Add ability to test Ironic UI with Horizon master branch
For the developers needs it's useful to be able run tests with the
latest Horizon version from the master branch.
This patch introduces new tox environment py3-dev to run tests with the
Horizon master branch.
By default, Horizon will be installed from the master branch of it's git
repository. You can override this behaviour by setting HORIZON_REPO
environment variable to test the plugin with your local copy.