Revisit support for state

[lauwers]

The TOSCA spec defines a set_state activity that can be used by policies and workflows. As far as I can tell, this is the only place in the spec where node state is referenced or used. We should revisit whether it is appropriate for the TOSCA language to have built-in support for state (in which it needs to be formally defined) or whether we should all references to state from the spec.