Updates Compute tests due to new behavior capabilities

* Simplified test_create_server due to the new
  create_server_with_defaults function
* Updated image tests to use create_active_image, which
  now properly watches the task state of the server being
  imaged.
* Corrected response code check to the correct code in
  test_config_drive_base
* Correct class variable name in rebuild_and_await

Change-Id: Id74ebd1668005d8319732048dc81f0b40ae33324
Merge "Updates Compute tests due to new behavior capabilities"