Template compatibility

In recent Puppet versions, warnings are shown whenever template
variables in ERB are accessed as local variables instead of instance
variables. Behaviour is identical as before.
Caching of self.ready

Performance improvement. Instead of checking cluster availability on
every crm call, we're only doing it the first time. This assumes the
cluster doesn't go down in the meanwhile, but the original check wasn't
atomic anyway so this is just a nice performance boost.