Handle clusters in core of crowbar

We have a runtime dependency on the pacemaker barclamp here. If it's not
available, then we simply won't support clusters at all. If it's
available, we will use it to resolve clusters.
Integrated cluster selection into node selector
Add helpers to get hostnames for OpenStack endpoints

This is common code that all barclamps need, so a helper is a good
thing.
Understand cluster in template that we don't use

Just so we can say we've covered everything :-)