ceph_key: add fetch_initial_keys capability

ceph_key: add fetch_initial_keys capability

This is needed for Nautilus since the ceph-create-keys script goes away.
(https://github.com/ceph/ceph/pull/21305)
Now the module if called with 'state: fetch_initial_keys' will lookup
keys generated by the monitor and write them down on the filesystem to
the right location (/etc/ceph and /var/lib/ceph/boostrap*).
This is not applicable to container since keys are generated by the
container only.

Signed-off-by: Sébastien Han <seb@redhat.com>