Postgres is used by several roles, but the setup is currently part of the 'mailserver' role. By moving it to 'common', it's possible to disable the mailserver without breaking the others.

Clean up some artifacts from the merge to match the `jessie` branch.
Keep the changes on master made with commits bfe6fe84 and e229c551.

The `private_key` variable used to check for changes was never
registered.

While adding the Let's Encrypt offline testing block in 1746afcc we
accidentially duplicated a the 'when' statement. Ansible only looks at
the last when statement for a given block meaning the earlier one has no
use. This commit merges both lines in one.

See https://docs.ansible.com/ansible/playbooks_lookups.html