ceph_volume_client: set an existing auth ID's default mon caps

ceph_volume_client: set an existing auth ID's default mon caps

... as 'allow r' (the minimum mon caps required to access a share)
when:

* authorizing the auth ID to access a volume.

* deauthorizing the auth ID to access a volume, but the auth ID is
  authorized to access other volumes.

In both the above cases, the ceph_volume_client previously tried to
set the mon caps of the auth ID to an invalid value, None.

Fixes: http://tracker.ceph.com/issues/17800
Signed-off-by: Ramana Raja <rraja@redhat.com>