Add unit tests for verifying extensions list

This commit adds unit tests for verify extensions functionality in
verify_tempest_config.py. It covers the use with all the currently
supported services: nova, cinder, nova_v3, neutron, and swift.

Change-Id: Idbc25ec95f1d2697dbf1994e81b83938b2e85a9f
Merge "Add unit tests for verifying extensions list"