Nose has been in maintenance mode for the past several years. It has
issue with exit code [1] which leads to false positive results for our
seleniun-headless job.

This patch changes test runner for Horizon tests and does the following
things:

* Django test runner  executes test in a different order than Nose does.
  That's why we've got an issue with side-effect in
  horizon.tests.unit.tables.test_tables.MyToggleAction class. This patch
  adds workaround to it.
* Rename filename of test files to names starting with 'test_'
  so that the django test runner can find tests expectedly.
* '--with-html-output' option is temporary dropped and will be added in
  a following patch.
* Integraion tests is marked via django.test.tag mechanism which is
  introduced in Django 1.10
* 'selenium-headless' is broken now because we don't have geckodriver on
  gates, this patch makes it non-voting.
* 'tox -e cover' is fixed
* Remove @memorized decorator from
  dashboards.project.images.images.tables.filter_tenant_ids function.

[1] https://github.com/nose-devs/nose/issues/984

Depends-On: https://review.openstack.org/572095
Depends-On: https://review.openstack.org/572124
Depends-On: https://review.openstack.org/572390
Depends-On: https://review.openstack.org/572391

Related blueprint: improve-horizon-testing
Change-Id: I7fb2fd7dd40f301ea822154b9809a9a07610c507