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