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: Iaa6026a504cc53784aad5731e9afe8b684b3ede5

dtroyer/os-client-config,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,stackforge/python-openstacksdk,redhat-openstack/os-client-config,dtroyer/python-openstacksdk,openstack/os-client-config,openstack/python-openstacksdk,openstack/python-openstacksdk,switch-ch/os-client-config