mgr/cephadm: 'ceph orch client-keyring ...' commands to manage keyring files

mgr/cephadm: 'ceph orch client-keyring ...' commands to manage keyring files

Teach cephadm to manage keyring files on cluster hosts.  These keys must
already exist in the mon auth database--cephadm does not create them if
they don't exist (and will issue warnings to the log if they do not).

A ceph.conf is pushed implicitly along with the keyring file.

Each keyring added will be pushed to the hosts described by the placement
spec with the appropriate ownership and mode.  If the ownership, mode, or
path are modified, the files are rewritten or removed as need.

If the client-keyring entry is removed, the keyring files are removed.

Signed-off-by: Sage Weil <sage@newdream.net>