ceph_key: support using different keyring

ceph_key: support using different keyring

Currently the `ceph_key` module doesn't support using a different
keyring than `client.admin`.
This commit adds the possibility to use a different keyring.

Usage:
```
      ceph_key:
        name: "client.rgw.myrgw-node.rgw123"
        cluster: "ceph"
        user: "client.bootstrap-rgw"
        user_key: /var/lib/ceph/bootstrap-rgw/ceph.keyring
        dest: "/var/lib/ceph/radosgw/ceph-rgw.myrgw-node.rgw123/keyring"
        caps:
          osd: 'allow rwx'
          mon: 'allow rw'
          import_key: False
        owner: "ceph"
        group: "ceph"
        mode: "0400"
```

Where:
`user` corresponds to `-n (--name)`
`user_key` corresponds to `-k (--keyring)`

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>