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

...ot

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
since we're checking in rmsnap now, let's change this back to _snap_?  :)

otherwise, ready to merge!