ensemble status should take into account unit and relation workflows and unit agent status

[Kapil Thangavelu]

The topology reflects planned state, not nesc. actual state. For entities with associated agents, it would be useful to display the presence of their agents.

[Kapil Thangavelu (hazmat)]

As well as for unit relations to show the relation status.

[Jim Baker (jimbaker)]

Per conversation on #ubuntu-ensemble:

Unit anrd relation workflow is currently written to ZK for global observability. (And tested, see test_workflow.WorkflowTestBase.read_persistent_state) . However, there's no way to currently construct what that state is. Kapil recommends subclassing UnitWorkflowState and RelationWorkflowState such that they be instantiated from ZK instead of the local file store. ensemble status can then interrogate these state objects much like it does for other aspects of status.

[Kapil Thangavelu (hazmat)]

also such implementations should not allow storing, ie they need to override _load/_store. _load to read from zk, and _store to raise an error. multiprocess usage of the workflow in r/w fashion requires additional coordination.