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

This is done to suppress warnings from ansible-lint.

Depending on when the client is run, there are no certificates to
update.  By default, the client runs in interactive mode and wants to
notify the user of this.  This causes Ansible to hang waiting for an
acknowledgement that will never come.  Adding the non-interactive flag
fixes this.