This is a migration step to utilize tempest plugin
system instead of directly calling functional api tests.
This is the approach used by a number of other projects as well
as an approved process by openstack-qa.
The difference in execution is that we will need to execute tempest's
tox instead of our own:
tox -eall-plugin magnum.tests.functional.api.v1 -- --concurrency=1
Implements: blueprint magnum-tempest
Change-Id: Ic3eadae7fb5d88b776f9ded9589ef25279a2e1be
63de8e5540