Document how to run the tempest tests

This patch is extending the developer quick-start guide to include
information about how one can run the integration tests (tempest) against
a running devstack cloud (the setup for such a cloud is already part of
the documentation).

The patch also removes the information about how to run tests from the
ironic_tempest_plugin/README.rst file and add a link pointing to our
documentation.

Change-Id: I6aeb3a4fa735ff08cbb62b0b15cf95be57d1a308
1 file changed