mon/MDSMonitor: add mon auth caps for CephFS names

mon/MDSMonitor: add mon auth caps for CephFS names

Add a 'fsname' clause to mon auth caps to restrict a client's view
of the FSMap. Example:

mon 'allow rw fsname=cephfs2'

This would restrict the client's view of the FSMap to the MDSMap for
cephfs2. Any MDS allocated to a different filesystem will be invisible.
Global standby daemons are always visible. To allow multiple CephFSs,
add multiple caps:

mon 'allow rw fsname=cephfs2, allow rw fsname=cephfs2'

Fixes: http://tracker.ceph.com/issues/15070
Signed-off-by: Douglas Fuller <dfuller@redhat.com>
Signed-off-by: Rishabh Dave <ridave@redhat.com>