add installation of postgresql-contrib for uuid-ossp, and fix reporting of uuid-ossp as "changed" on each run.

I tried to use this role to setup postgresql, and it worked great except for the `uuid-ossp` part (it needs the `postgresql-contrib` package installed). Also, running `ansible-playbook` on hosts that have already had the postgresql role applied successfully will result in Ansible reporting the `uuid-ossp` installation as having changed (even though the output says `NOTICE:  extension "uuid-ossp" already exists, skipping`

This PR includes:
- BUGFIX: added `postgresql-contrib` to the list of apt packages to install for uuid-ossp support
- ENHANCEMENT: added checks on "Add uuid-ossp..." task within `databases.yml` so Ansible won't incorrectly report the uuid extension installation as "changed" on each run.