To give more flexibility for tempest test coverage, use the tempest plugin
interface to add tempest tests to the Cinder repo.
Just the plumbing for now. A temporary tempest test was added just for
validation. This test should be removed once additional tests are added.
Third party CIs should change their command line from:
tox -e all -- volume
To something like:
tox -e all-plugin -- volume
Change-Id: I679b1cee6dc9187fd2b11c47a14be9f7acdad938
Implements: blueprint add-intree-tempest-tests
b906aee01d