Add PostgreSQL 11.X as a third database version

We need to start including PostgreSQL 11.X as part of the omnibus-gitlab package. Current latest version is 11.6. We should track new versions of 11.X during the work on this issue, and switch to newer versions if appropriate.

We currently include 9.6.x, and 10.X. We install 10.X on new installations, and upgrade 9.6.X installations to 10.X automatically unless explicitly opted out. This should not change initially.

The end state of this issue should be

1. 9.6.X could still be used if postgresql[version'] is explicitly set to 9.
2. 10.X will be the default version that is installed. Unless explicitly opted out, databases on version 9 will automatically upgrade to 10.X
3. 11.X will be an option for fresh installations
   1. It will be opt-in. Setting postgresql['version'] = 11 ahead of installation will do a new installation with version 11.
   2. We will handle automatic upgrades in a separate issue.
   3. We will verify that this works for different installation types (HA, GEO, etc) in a separate issue