ceph::mds: Rework key handling, tweak configuration

Use ceph::key instead of an exec.
Inject the key using client.admin.
Removed ceph::conf (this should be managed elsewhere)
Added ceph::conf::mds.
Merge pull request #8 from dmsimard/mds-rework-take2

MDS: Rework key handling, tweak configuration

Reviewed-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>