Add support for declarative policies

[lauwers]

TOSCA's current policy syntax supports imperative policies only. These policies follow an event-condition-action pattern. We should add support for declarative policies that follow a context-capability-constraint pattern and express invariant conditions that must hold true at all times. Ironically, all the policy examples in the spec (e.g. colocation and anti-colocation placement policies) are examples of declarative policies, but TOSCA currently does not include syntax that allows one to define the semantics of such policies.

[lauwers]

I envision declarative policies to be defined using a set of invariants, i.e. Boolean expressions that must hold true at all times. However, this will likely require extensions to our TOSCA Path syntax to allow get_property functions to access members of a group. For example:

groups:
  sw-group:
    members: [ sw1, sw2, sw3 ]

policies:
  anti-colocation:
    target: sw-group
    conditions:
      $not:
        $equal:
          - $get_property: [ ALL, RELATIONSHIP, host, TARGET, uuid ]
          - $get_property: [ OTHER, RELATIONSHIP, host, TARGET, uuid ]

In this TOSCA Path proposal, the ALL keyword means all members of the target group and OTHER means *all other members of the group`.

Alternatively, we could introduce a GROUP keyword to avoid the need to identify the target group in the policy definition. Comments?