Reuse v2 extension client for cinder v3

Now(Pike) Cinder versions are like
 - V1: Deprecated
 - V2: Deprecated
 - V3: Current

So users should be able to disable cinder v2 API on Tempest config.
This patch makes verify_tempest_config reuse v2 extension client for
v3 also. In addition, this patch removes v1 extension client usage
on verify_tempest_config because Tempest doesn't contain v1 tests
at all and we don't need to verify the v1 extensions at all.

Change-Id: I3ef5a82a70fa9e4b862f50a85f93351888ed6bcd
2 files changed