Plus some php cache tunings

Add role prefix to role specifig variable to avoid conflicts when
using multiples role on the same machine on the same play.

- Update to 18.0.4
- Version defined as variable in defaults/mail.yaml

Now it's possible to configure LDAP with servers that only support
plain-text authentication over insercure channels.

Role to configure nextcloud with LDAP User backend.

First test passed.

New modules:

- occ:
  set coniguration values using `php occ` nextcloud command-line tool.