mon/AuthMonitor: Add special syntax for cephfs

mon/AuthMonitor: Add special syntax for cephfs

Add a new command, ceph auth fs, to generate and apply auth caps for
cephfs.

Syntax:

  ceph auth fs <fs_name> <entity_name> (<dir> <cap>)...

Notes:

  In this case, the AuthMonitor will determine the correct osd and
  mds cap syntax based on the data pools used by <fs_name>. The same
  caps will be applied to all data pools regardless of the filesystem
  configuration. If the write cap is enabled on any directory, the write
  cap for every data pool will be obtained.

  If different auth caps for this entity already exist, the result is
  -EINVAL.

Fixes: http://tracker.ceph.com/issues/20885
Signed-off-by: Douglas Fuller <dfuller@redhat.com>
fixup