Refactoring v2.0 image fixtures.

Refactors fixtures:
  - move all behavior related methods to image behaviors.
  - add an image behavior property.

Depends on: https://review.openstack.org/#/c/53161/

Change-Id: I95209e4f6892d6b14a5b21348c0f384cffa4a02f
Merge "Refactoring v2.0 image fixtures."