ceph-fetch-keys role currently works only if cluster name is 'ceph'.
This commit allows to set custom cluster name in 'defaults' in the same
fashion as other roles do.
The purpose of this is so we can connect to the mons and gather the keys
needed to configure an OSD or additonal MON without having to reconfigure
the existing mons at the same time.
Signed-off-by: Andrew Schoen <aschoen@redhat.com>