Rework exceptions in Tempest

* Add base exception class similar to other OS projects
* Catch certain HTTP errors and raise exceptions in base
  client classes
* Fixes LP Bug#899701 by adding tearDownClass() method
  to the test_list_images.ListImagesTest class to destroy
  images and instances the test case creates

Change-Id: I0f616813539b31da27e5106a59c2ca3765b1919f
6 files changed