upstart, ceph-create-keys: Make client.admin key generation automatic.

upstart, ceph-create-keys: Make client.admin key generation automatic.

This should help simplify Chef etc deployments. Now (when using the
Upstart jobs), when a ceph-mon is started, ceph-create-admin-key is
triggered. If /etc/ceph/$cluster.client.admin.keyring already exists,
it does nothing; otherwise, it waits for ceph-mon to reach quorum, and
then does a "ceph auth get-or-create" to create the key, and writes it
atomically to disk.

The equivalent code can be removed from the Chef cookbook once this is
in.

Signed-off-by: Tommi Virtanen <tv@inktank.com>