Fixes LP Bug #912596 - image_ref_alt not found

* Adds skippers to tempest to enable skipping tests
  that don't meet certain conditions
* Adds a check in test_list_servers.ServerDetailsTest.setUpClass
  that checks if the image with ID image_ref_alt actually exists
  and if it does not, marks a specific test as skipped.

This is what the resulting test run looks like after adding
the skip:

jpipes@librebox:~/repos/tempest$ nosetests -sv tempest.tests.test_list_servers
Return the full details of a single server ... ok
Filter the detailed list of servers by flavor ... ok
Filter the detailed list of servers by image ... SKIP: Only one image found
Filter the detailed list of servers by server name ... ok
Filter the detailed list of servers by server status ... ok
Return a detailed list of all servers ... ok

----------------------------------------------------------------------
Ran 6 tests in 108.909s

OK (SKIP=1)

Change-Id: Ia6178ca8201acc03566b785ec096eda51d0425c9
2 files changed