Run tempest tests with testr

The primary reason for this is that test skips exceptions are
interpreted as skips not errors.

A secondary benefit is that testr revolves around tasks lists so we can
fine tune the task list without modifying tempest code.

Related:
  * XML tests failing: https://github.com/rcbops/ansible-lxc-rpc/issues/653
  * Tempest Testing: https://github.com/rcbops/ansible-lxc-rpc/issues/472