ceph-create-keys: update client.admin if it already exists

ceph-create-keys: update client.admin if it already exists

This means that anyone who de-privileges their client.admin
user will find that it gets resets with full privileges again.
This is necessary because we cannot distinguish between an old
key and a deliberately de-privileged key, and it's okay because
this is the admin key, it's meant to have access to everything.

Fixes: http://tracker.ceph.com/issues/19940
Signed-off-by: John Spray <john.spray@redhat.com>