Migrate certbot-auto users on CentOS 6 to Python 3.6 (#7268)

Fixes #7007

Python 3.4 is [EOL](https://www.python.org/dev/peps/pep-0429/), and only Python 3.x version available for CentOS 6 through EPEL is this version, and so is used by `certbot-auto`, the only official way to install Certbot on this platform.

This unpleasant situation becomes a little more uncomfortable, considering that the newest `pip` version (19.2) [just dropped Python 3.4 support](https://github.com/pypa/pip/issues/6685) and will refuse to start on this Python version. We can expect a lot of dependencies to follow this path now.

One direct result of this situation is that a fix to support correctly the ARM platforms requires to upgrade `pip` to 19.2 for `certbot-auto`. So this is not possible right now.

Then, let's upgrade Certbot instances on CentOS 6 to a supported version of Python 3.

This PR proposes a new bootstrap approach for CentOS 6 platform, `BootstrapRpmPython3Legacy`, that will install Python 3.6 from [SCL](https://www.softwarecollections.org) (the latest one available for now on CentOS 6). In term of Python 3 specific bootstrap methods, I take the occasion here to completely separate the bootstrap of CentOS 6 as a legacy system, from the RPM-based newest systems (like Fedora 29+) that are simply dropping support for Python 2.x. This is in prevision of future migration for all systems on Python 3.x, that is a different problematic than supporting old systems.

* Add logic

* Rebuilt letsencrypt-auto

* Fix logic

* Focus on specific packages

* Maintain PATH for further invocations of letsencrypt-auto after bootstrap.

* Various corrections

* Fix farm test for RHEL6

* Working centos6 letsencrypt-auto self tests

* Fix test_sdist for CentOS 6

* Corrections

* Work in progress

* Working configuration

* Fix typo

* Remove EPEL. Add a test.

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Improvements after review

* Improvements

* Add a comment

* Add a test

* Update a test

* Corrections

* Update function return

* Work in progress

* Correct behavior on oracle linux 6.

* Corrections

* Rebuild script

* Add letsencrypt-auto tests for oraclelinux6

* Update tox.ini

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Remove specific code for scientific linux

* Change some variables names

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Various corrections

* Fix tests

* Add a comment

* Update message

* Fix test message

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update scripts

* More focused assertion

* Add back a test

* Update script

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Check quiet mode

* Add changelog

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

tox.ini

@@ -206,7 +206,17 @@ passenv = DOCKER_*
 # At the moment, this tests under Python 2.6 only, as only that version is
 # readily available on the CentOS 6 Docker image.
 commands =
-    docker build -f letsencrypt-auto-source/Dockerfile.centos6 -t lea letsencrypt-auto-source
+    docker build -f letsencrypt-auto-source/Dockerfile.redhat6 --build-arg REDHAT_DIST_FLAVOR=centos -t lea letsencrypt-auto-source
+    docker run --rm -t -i lea
+whitelist_externals =
+    docker
+passenv = DOCKER_*
+
+[testenv:le_auto_oraclelinux6]
+# At the moment, this tests under Python 2.6 only, as only that version is
+# readily available on the Oracle Linux 6 Docker image.
+commands =
+    docker build -f letsencrypt-auto-source/Dockerfile.redhat6 --build-arg REDHAT_DIST_FLAVOR=oraclelinux -t lea letsencrypt-auto-source
     docker run --rm -t -i lea
 whitelist_externals =
     docker