convert test runner to tox

We now have tox definitions for the test runs for tempest, which
lets us control the gate commands from tempest. Use tox to run
tempest in the gate now.

Part of bp:tempest-repo-restructure

Change-Id: Idc775c3caff0284da8d7f735d76b3e50e39bb365
Merge "convert test runner to tox"