Read the bootstrap-osd key for `cluster`.

def get_bootstrap_osd_key(cluster):
    """
    Read the bootstrap-osd key for `cluster`.
    """
    path = '{cluster}.bootstrap-osd.keyring'.format(cluster=cluster)
    try:
        with open(path, 'rb') as f:
            return f.read()
    except IOError:
        raise RuntimeError('bootstrap-osd keyring not found; run \'gatherkeys\'')