cinder v2 api tests - fix volume client

This is a legacy problem in change
I712a90d39981564af09393ed1b7f4be12897c808

1. move cls.volumes_client, cls.volumes_extension_client
   cls.snapshots_client from BaseVolumeTest into
   subclass(BaseVolumeV1Test)
2. delete call cls.volumes_client.keystone_auth as it is
   called automatically in request method of RestClient.

Change-Id: I5184de9911e3fe9a3c23c2c4f84e59596f10b77c
Implements: blueprint cinder-v2-api-tests
Update README.rst with details about unit tests

This commit just fixes a couple of issues with the README file and adds
some details about the OS_TEST_PATH env variable and unit tests.

Closes-bug: #1268440

Change-Id: I17f75f7d7b3a2ac75eac884bc403bc46d6a2e77f
Merge "cinder v2 api tests - fix volume client"
Merge "Update README.rst with details about unit tests"