Expanding tests for le-auto, adding CentOS test suite (#2671)

* Adding Dockerfile for CentOS 6

* Adding CentOS 7 to LEA tests

* Enabling CentOS 6 LEA test

* Removing CentOS 7

* Fixing nits

* Using yum to install epel-release

.travis.yml

@@ -71,6 +71,11 @@ matrix:
       services: docker
       before_install:
       addons:
+    - sudo: required
+      env: TOXENV=le_auto_centos6
+      services: docker
+      before_install:
+      addons:
     - python: "2.7"
       env: TOXENV=apacheconftest
       sudo: required