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
diff --git a/tempest/api/compute/keypairs/base.py b/tempest/api/compute/keypairs/base.py
index 76e5573..15f231b 100644
--- a/tempest/api/compute/keypairs/base.py
+++ b/tempest/api/compute/keypairs/base.py
@@ -16,11 +16,9 @@
 from tempest.api.compute import base
 
 
-class BaseKeypairTest(base.BaseComputeTest):
+class BaseKeypairTest(base.BaseV2ComputeTest):
     """Base test case class for all keypair API tests."""
 
-    _api_version = 2
-
     @classmethod
     def setup_clients(cls):
         super(BaseKeypairTest, cls).setup_clients()