add ceph-mgr to luminous release

Some detail; the charm will need to generate a new keyring for each -mgr daemon and place it into /var/lib/ceph/mgr/ceph-`hostname`/keyring (which must be ceph:ceph owned) - there is a special profile for this key - see [0].

The ceph-mgr instance can then be enabled and started:

  systemctl enable ceph-mgr@`hostname`
  systemctl start ceph-mgr@`hostname`

this needs to happen post mon bootstrap on each unit.

[0] http://docs.ceph.com/docs/master/mgr/administrator/

Reviewed: https://review.openstack.org/491917
Committed: https://git.openstack.org/cgit/openstack/charm-ceph/commit/?id=94e2db92e12209b3b87da472f783d39e652681d2
Submitter: Jenkins
Branch: master

commit 94e2db92e12209b3b87da472f783d39e652681d2
Author: Corey Bryant <email address hidden>
Date: Tue Aug 8 20:06:30 2017 +0000

    Bootstrap ceph-mgr daemon

    As of the Luminous release, Ceph requires ceph-mgr daemons be run
    to have a fully functional cluster. [0]

    [0] http://docs.ceph.com/docs/master/mgr/administrator/

    Change-Id: I59e4d20c0e3c2d3e4cff45aaa70adf5b2862f71b
    Closes-bug: #1709061

Reviewed: https://review.openstack.org/491918
Committed: https://git.openstack.org/cgit/openstack/charm-ceph-mon/commit/?id=3304c79b745796f4567ab1b9d8d501d3974d3362
Submitter: Jenkins
Branch: master

commit 3304c79b745796f4567ab1b9d8d501d3974d3362
Author: Corey Bryant <email address hidden>
Date: Tue Aug 8 20:42:01 2017 +0000

    Bootstrap ceph-mgr daemon

    As of the Luminous release, Ceph requires ceph-mgr daemons be run
    to have a fully functional cluster. [0]

    [0] http://docs.ceph.com/docs/master/mgr/administrator/

    Change-Id: Ie8c13f3e7ada000e074d2c4f7d996872cd3ceeef
    Closes-bug: #1709061