corosync: Add attribute to not start corosync on boot after fencing

If the node[:corosync][:require_clean_for_autostart] attribute is set to
true, then corosync will not start on boot automatically, nor through
chef-client, if the file /var/spool/corosync/block_automatic_start
exists.

On proper shutdown, one new init script (corosync-shutdown) is used to
remove that file and stopping corosync, resulting in the next boot to
have corosync starting.

This allows to not start corosync after fencing, to avoid a never-ending
deathmatch, or a fencing loop.
Expose require_clean_for_autostart attribute in crowbar
Add "automatic" option in crowbar for require_clean_for_autostart

By default, we want to use require_clean_for_autostart for a two-nodes
cluster, and only in that case. This is something that we do at the
orchestration layer with crowbar, by wrapping this value in a tri-state
attribute.
Fix potential use of undefined variable