Refactor taskmanager. Split heat/native/migration code path

Reasons:
 - Trove should be able to manage resource provisioning through:
     - nova, cinder, designate etc. (navites);
     - heat. (orchestration).
     - Main reason is to provide harmless migration from the nova to heat,
       when heat would be cover all required use cases such as
       provisioning/resizing/network-attributes-management.

    Changes:
 - Refactoring:
   - dividing actual manager onto navite(nova/cinder/designate/neutron)
     and orchestrator(heat);
   - extracting common code into taskmanager/common.py
   - fixing tests;
   - Resize instance for orchestrator added;
   - Resize volume for orchestrator added.

Change-Id: Ibdab5cb78bc2a83112fe9813b7effd6e0dd0a503
Implements: blueprint resource-manager-interface