Remove requirements.txt from tox.ini
Remove requirements.txt from tox.ini

From lifeless: pbr reflects the package dependencies from
requirements.txt into the sdist that tox builds.

Change-Id: I0590548763d27a8806cd1e0e5c3f2df95cda2bc7

dtroyer/python-openstacksdk,dudymas/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,mtougeron/python-openstacksdk,briancurtin/python-openstacksdk,stackforge/python-openstacksdk,mtougeron/python-openstacksdk,briancurtin/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dudymas/python-openstacksdk