Autogenerate initial mon key

As mentionned in the issue 24 it's not really safe to store a default
fsid nor a monitor key. Thus the commit brings the auto-generation of
the initial monitor key. However it is quite complex to do the same for
the fsid, so I leave this to the person in charge of the deployment to
generate one and edit group_vars/all accordingly. The default fsid has
been removed as well.

Close: #24

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>