extend functional tests

Depends on:
Change-Id: I86a280cf3d186876da9dbd44c8099aac87cf56c3

 - now testing for disjoint sets in ListImagesTest instead of previous
   shenanigans.
 - rebased with master and split tests into separate modules
 - moved _setup_test_data as a fixture for ListImagesTest
 - removed unused imports
 - refactored Image model to use renamed property id_
 - rebase and removed hardcoded urls
 - added more tests to be on par with tempest

Change-Id: If3346371a0e3e43490cad0f322f7913a64a3fd8f