Adds subscription groups and simplifies the interaction between scheduler and control

[jcooklin]

Fixes #832, #852, #1052
Closes #997

Simplified interaction between scheduler and control. Introduces new subscription mechanism with task view. Plugin loading/unloading triggers subscribtion/unsubscribtions from pools.

Link to design document

Summary of changes:

• subscriptionGroups in pluginControl for storing (requests, expanded metrics, plugins and configuration)
• No more expanding metrics on each collect job
• ValidateDeps takes care of expanding metrics when task is created
• subscriptionGroups are added/removed when tasks are started/stopped (SubscribeDeps/UnsubscribeDeps)
• CollectMetrics leverages subscriptionGroups
• subscriptionGroups are processed on plugin load/unload events
• No need for bound vs unbound subscriptions with respect to pools

scenarios

• [upgrade collector] load plugin mock v1, start task mock-file.yaml (not pinned), load mock v2
• Expected mock v1 is unsubscribed (not running), mock v2 is subscribed (running)
  
• [downgrade collector] load plugin mock v2, start task mock-file.yaml (not pinned), load v1
• Expected mock v2 is subscribed and running, mock v1 is loaded (not running)
  
• [a dynamic metric request] load plugin mock v1, start task dynamic-mock-file.yaml, load another-mock
• Expected another-mock is subscribed to by our task
  

Testing done:

• unit tests
• manual

@intelsdi-x/snap-maintainers