Add stack_id to base instance model

Reasons:
- saving stack id would give to trove an
  ability to perform actions over stacks
  such as stack_update for resize;

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