Add stack_id to base instance model

Reasons:
 - saving stack id would give to trove an
   ability to perform actions such as
   stack update/delete;

Changes:
 - adding stack_id column into base schema;
 - DBInstance model update.

Change-Id: I193c15e72f93cc3fe5be72a41a1ad23d93e04047
Implements: blueprint stack-id
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;

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