allow running both images api versions

Removed dependancy on ImagesConfig.api_version so that tests can be run
for both v1 and v2 without changing config file.

Also separated v1 test fixture outside of images.fixtures module.

Renamed some classes for consistency and moved helper methods from
fixture to images behaviors.

Refactored to acommodate new get_all_tenants_id()

Change-Id: Ic829eb63cda6bb64400239b7c90978239393a4b8
Merge "allow running both images api versions"