update docs to mention the pacemaker fence agent based on powerman

[ofaaland]

pacemaker, a high availability package similar to heartbeat, depends on the ability to "fence" nodes (power them off or separate them from the network and/or storage) to safely fail a service over from one host to another.

A "fence agent" which uses powerman to do this exists and lives in https://github.com/ClusterLabs/fence-agents

There should probably be some mention of it in the powerman documentation.