Added db API layer for CRUD operations on instance tags

Added database API layer which allow following:
- add tag to the instance
- replace set of instance tags with new set of tags
- get all instance tags
- remove specified tag from instance
- remove all tags from instance

Implements: blueprint tag-instances
Change-Id: I43b5b22dff79b14f352ff511f8815fc81b14b923
Merge "Added db API layer for CRUD operations on instance tags"