Update pypi mirror to use pypi.dfw.openstack.org

The OpenStack infrastructure's PyPI mirror (pypi.openstack.org) shouldn't
be used for 3rd parties so setting the default to pypi.dfw.openstack.org
instead.

Change-Id: I7720ad7d3b23816c63e971112a3a733bde52b7f4
Merge "Update pypi mirror to use pypi.dfw.openstack.org"