Add tox env to build docs

In django-based projects, we need to specify DJANGO_SETTINGS_MODULE
when running docs build. It is required to addresses the warnings of
docs build job in the gate.

This commit also changes run_tests.sh --docs to use "python setup.py
build_sphinx" to match tox docs env. This change addresses all sphinx
build warning except keystone v3 related following warning.
I believe it is useful.
  NoReverseMatch: u'domains' is not a registered namespace
  inside 'horizon:admin'

Closes-Bug: #1302532
Related-Bug: #1257725

Change-Id: I3663b81b58be3c6cb05e4af914f768c28b21ea07
Better tab name on Hosts Aggregate modal.

Change the Tab name on "Create Host Aggregate"
modal for a better match with the action.

Change-Id: I87d1ae63d719afcb6d5f9827171f8392d6dc3a8d
Closes-Bug: #1291120
Return False if instance is None in TogglePause and ToggleSuspend

Change-Id: Iad67cc61954f57c4260467d3f827a51b932b74b9
Closes-Bug: #1306216
Merge "Better tab name on Hosts Aggregate modal."
Merge "Return False if instance is None in TogglePause and ToggleSuspend"
Merge "Add tox env to build docs"