create a `{cluster}.mon.keyring` file if it doesn't exist

So that we don't halt execution when deploying monitors, requiring a user to call `new`:

    $ceph-deploy mon create-initial
    [ceph_deploy.conf][DEBUG ] found configuration file at: /Users/alfredo/.cephdeploy.conf
    [ceph_deploy.cli][INFO  ] Invoked (1.5.17): /Users/alfredo/.virtualenvs/ceph-deploy/bin/ceph-deploy mon create-initial
    [ceph_deploy.mon][WARNIN] keyring (ceph.mon.keyring) not found, creating a new one
    ...



Reference ticket: http://tracker.ceph.com/issues/9662