EE correspondence of gitlab-org/gitlab-ce!10156

Merge branch 'test-pg-mysql' into tmp

* test-pg-mysql:
  Properly use $BUNDLE_INSTALL_FLAGS
  Always use . and add more comments to prepare_build.sh
  Remove duplicated line
  Avoid Rails generating complicated query, see:
  Move variables back to where they are
  Just set GITLAB_DATABASE in the script
  Add # host for development and test as well
  We cannot use array in yaml variables
  Set profiler only for postgresql not mysql
  Use GITLAB_DATABASE: $CI_JOB_NAME[1] so that we
  Replace on host rather than socket, feedback:
  Only upload/download knapsack reports from pg
  Switch back to original repository
  Test all on EE, only master on CE, and -mysql branches
  Note that install knapsack later than bundle install
  Test both PostgreSQL and MySQL for the win.