Add actionability parameter to the rule baseclass.

[Max-Huneshagen]

We add an actionability parameter to the base class rule. If True, this parameter indicates that the non-fulfilment of the rule can be solved without recreating any resource and/or stack. The standard value is set to True for all rules.

[jsoucheiron]

Maybe it'd be worth to add something a bit more extensible. We could add a variable called SOLUTION_TYPE it could be an enum with a few values: REQUIRES_CF_TEARDOWN, REQUIRES_RESOURCE_RECREATION, IN_PLACE_UPDATE...