Remove BaseComputeTest and _api_version

On the history Nova had two major API versions V2 and V3, and Tempest
contained the tests for both APIs with inheritances of test classes.
Then some classes contain _api_version and there are some base test
classes.
However V3 has been removed and now these base test classes and the
_api_version are meaningless. So it is nice to remove for cleanup.

Change-Id: Icefb304a94492d92905cc425c442f10ebfe7d8b3
8 files changed