Add CrowbarPacemakerHelper.is_cluster_founder? helper

This will avoid other cookbooks having to know about implementation
details for determining if a node is a founder or not.
Add CrowbarPacemakerHelper.cluster_founder helper

This will avoid other cookbooks having to know about implementation
details for finding the founder.
Merge pull request #52 from vuntz/is-founder

Add helpers to hide implementation details for other cookbooks