Add a STONITH recipe

By default, STONITH is marked as disabled. It can be enabled and
configured:

 - manually (by the user)
 - with a stonith plugin that will be used with a clone resource
 - with a stonith plugin that will have per-node parameters
Add support for IPMI as STONITH in crowbar-pacemaker

Since Crowbar can know about IPMI configuration of nodes, we can
translate this into a valid STONITH configuration.
Compute no-quorum-policy based on number of members in the cluster

This is "ignore" if we have 1 or 2 members, and "stop" (the default) for
more members.

We do this magic in the crowbar-pacemaker cookbook, since we have the
required information there.
Check that the specified STONITH plugin is available

We simply need to check if the plugin is in the output of "stonith -L".
Add support for using libvirt STONITH plugin in crowbar-pacemaker

For development purposes, it's very useful to be able to simply use the
external/libvirt STONITH plugin. It turns out that we only need to be
aware of the hypervisor IP address; all the other information we can
find through node attributes.

Useful resource:
http://www.hastexo.com/resources/hints-and-kinks/fencing-libvirtkvm-virtualized-cluster-nodes
Add attributes for STONITH
Add UI bits for libvirt mode
Add validation for stonith bits of the proposal
Validation for libvirt
Generate hostlist for clone stonith