Explicitly use django_nose.NoseTestSuiteRunner

Horizon is going to switch to Django Test Runner [1].
To not break Watcher Dashboard we need to use Nose Test Runner
until we'll be ready to remove Nose from watcher-dashboard.

[1] https://review.openstack.org/#/c/544296/

Change-Id: Ia8a6c185cfea09e476c2f5fbe134cc84f450c096

watcher_dashboard/test/settings.py

@@ -32,6 +32,8 @@ INSTALLED_APPS = (
 import openstack_dashboard.enabled
 import watcher_dashboard.local.enabled
 
+TEST_RUNNER = 'django_nose.NoseTestSuiteRunner'
+
 INSTALLED_APPS = list(INSTALLED_APPS)  # Make sure it's mutable
 settings_utils.update_dashboards(
     [