Solved offline nodes issue

CrowbarPacemakerHelper::cluster_nodes got extended is now able to only
return nodes that are currently online by crowbar timeouts. This should
solve the problem we run into, when a node in the cluster is offline. A offline
node will never transition to finished and cause the wait_for_mark method to
timeout.

The error window is reduced to the time frame when nodes go offline within 20
minutes before the chef-client run.