mon: add ceph osd pool set <pool> auid

When a pool is created with ceph osd pool create, the auid is not
inferred from the session auid and is set to zero. Add the

  ceph osd pool set <pool> auid <int>

command to set it after it is created, and the matching get:

  ceph osd pool get <pool> auid

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1717 from dachary/wip-auid

mon: add ceph osd pool set <pool> auid

Reviewed-by: Greg Farnum <greg@inktank.com>