Move nginx_compat to nightly (#7001)

With the various optimizations already done and upcoming (certbot-ci), the time execution of integration tests have significantly decreased, allowing potentially a complete execution of a Travis PR job to be done within 5min30s.

However, one job is significantly longer that the other ones after this migration: this is nginx_compat, that takes more that 11min to finish. I tried to split the nginx_compat in terms of tested configuration and of tests to execute (auth, install, enhance). Both are not satisfactory:

splitting by configuration may work, but add a significant complexity in the tests
splitting by tests type is supported almost out-of-the-box, but fails to make two fast tests (see https://travis-ci.org/adferrand/certbot/builds/525892885?utm_source=github_status&utm_medium=notification for instance)
Since these tests are designed to check corner cases on the nginx parser, this is mostly useless to execute them on each PR, as the nginx parser is rarely updated.

After some discussion with @bmw, I think that we can just move the nginx_compat from the PR tests to the nightly tests. This PR does that.

.travis.yml

@@ -41,12 +41,6 @@ matrix:
     - python: "2.7"
       env: TOXENV=py27-cover FYI="py27 tests + code coverage"
 
-    - sudo: required
-      env: TOXENV=nginx_compat
-      services: docker
-      before_install:
-      addons:
-      <<: *not-on-master
     - python: "2.7"
       env: TOXENV=lint
       <<: *not-on-master
@@ -94,6 +88,12 @@ matrix:
       <<: *not-on-master
 
     # Extended test suite on cron jobs and pushes to tested branches other than master
+    - sudo: required
+      env: TOXENV=nginx_compat
+      services: docker
+      before_install:
+      addons:
+      <<: *extended-test-suite
     - python: "2.7"
       env:
         - TOXENV=travis-test-farm-apache2