mds: Make min/max UID configurable for who is allowed to create a snapshot

This way administrators can configure which UIDs are allowed to create
snapshots on CephFS.

In certain situations it might be desirable that regular users are not allowed
to create snapshots. For example on shared hosting environments.

Fixes: #9029
mds: Also check min/max uid on snap removal

Not only check if the uid is correct when creating a snapshot, but also
verify if the UID is correct when trying to remove a snapshot.

Fixes: #9029
mds: Revert from mds_mksnap_ setting to mds_snap_ settings
Merge pull request #2222 from ceph/wip-9029

mds: Make min/max UID configurable for who is allowed to create a snapsh...

Reviewed-by: Sage Weil <sage@redhat.com>