Manila mount Ceph version confusion

We can no longer override `os_manila_mount_ceph_version ` at site.yml runtime, as the repos are disabled. We should skip the install task unless explicitly requested.

Also, let's remove this line as it's not representative of the defaults anymore: https://github.com/stackhpc/ansible-slurm-appliance/blob/main/environments/common/inventory/group_vars/all/manila.yml#L13