Fix H301 and H304 hacking issues

H301 one import per line                                                                            |
H304 No relative imports

When checking imports DJANGO_SETTINGS_MODULE environment needs to
be set. Add the following to tox.ini testenv:pep8.

    setenv = DJANGO_SETTINGS_MODULE=openstack_auth.tests.settings

A part of blueprint openstack-hacking-compliant

Change-Id: I65a23c1e9a5d7a5852d448651254b6a3866f1dd3
Merge "Fix H301 and H304 hacking issues"