Allow ceph service systemd overrides to be specified

Allow ceph service systemd overrides to be specified

ceph services can fail to start under certain circumstances (for
example, when running in a container) because the default systemd
service configuration causes namespace issues.

To work around this we can override the system service settings by
placing an overrides file in the ceph-<service>@.service.d directory.
This can be generic so as to allow any potential changes required to
the ceph-<service> service files.

The overrides file is only setup when the
"ceph_<service>_systemd_overrides" config_template override variable is
specified.

The available service systemd override files are as follows:
ceph_mds_systemd_overrides
ceph_mgr_systemd_overrides
ceph_mon_systemd_overrides
ceph_osd_systemd_overrides
ceph_rbd_mirror_systemd_overrides
ceph_rgw_systemd_overrides