tests: Add a couple of Selenium tests

tests: Add a couple of Selenium tests

While developing the new series UI, several bug crept in but weren't
discovered until later. All because we don't have in-browser tests to go
along the lower level tests we already have.

In particular, behaviours that need javascript to run cannot be tested
outside of a full environment with the pages being served to an actual
browser.

This commit introduces selenium to the test suite and starts with 2
simple tests to give a taste of what it looks like.

test_default_focus: make sure we do focus the username field on the
                    login page

test_login: shows how to chain actions to test the full login phase.
            This is quite similar the lower level test, except it also
            checks we display the username once logged in.

v2: Use LiveServerTestCase for django pre-1.7
v3: Propagate the DISPLAY environment variable to have an X display
    specified for the browser
v4: Log execution of the chrome driver, useful for debugging
v5: Rebase on top of upstream
v6: Pass environmental variables to tox and ensure PEP8 compliance

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>