Plus some php cache tunings

Don't use random uuid as internal user refs for LDAP users, otherwise
administration from the web interface become impossible.

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.