Stop using staging in apacheconftests (#6647)

Fixes #6585.

I wrote up three suggestions for fixing this at https://github.com/certbot/certbot/issues/6585#issuecomment-448054502. I took the middle approach of requiring the user to provide an ACME server to use. I like this better than the other approaches which were:

> Resolve #5938 instead of this issue.

There is value in these tests as is over the compatibility tests in that they don't use Docker and run on different OSes.

> Spin up a local Python server to return the directory object.

Trying to set up a dummy ACME server seemed hacky and error prone.

Other notes about this PR are:

* I put the Pebble setup in `tox.ini` rather than `.travis.yml` as this seems much cleaner and more natural.
* I created a new `tox` environment called `apacheconftest-with-pebble` that reuses the code from `testenv:apacheconftest` so `apacheconftest` can continue to be used with servers other than Pebble like is done in our test farm tests.
* I chose the environment variable `SERVER` for consistency with our integration tests. I chose to not give this environment variable a default but to fail fast when it is not set.
* I ran test farm tests on this PR and they passed.

certbot-apache/certbot_apache/tests/apache-conf-files/apache-conf-test

@@ -3,6 +3,11 @@
 # A hackish script to see if the client is behaving as expected 
 # with each of the "passing" conf files.
 
+if [ -z "$SERVER" ]; then
+    echo "Please set SERVER to the ACME server's directory URL."
+    exit 1
+fi
+
 export EA=/etc/apache2/ 
 TESTDIR="`dirname $0`"
 cd $TESTDIR/passing
@@ -56,13 +61,16 @@ if [ "$1" = --debian-modules ] ; then
     done
 fi
 
+CERTBOT_CMD="sudo $(command -v certbot) --server $SERVER -vvvv"
+CERTBOT_CMD="$CERTBOT_CMD --debug --apache --register-unsafely-without-email"
+CERTBOT_CMD="$CERTBOT_CMD --agree-tos certonly -t --no-verify-ssl"
 
 FAILS=0
 trap CleanupExit INT
 for f in *.conf ; do 
     echo -n  testing "$f"...
     Setup
-    RESULT=`echo c | sudo $(command -v certbot) -vvvv --debug --staging --apache --register-unsafely-without-email --agree-tos certonly -t 2>&1`
+    RESULT=`echo c | $CERTBOT_CMD 2>&1`
     if echo $RESULT | grep -Eq \("Which names would you like"\|"mod_macro is not yet"\) ; then
         echo passed
     else