Stop hard-coding the OpenStack PyPI mirror.

os-svc-install doesn't need to hard-code the OpenStack PyPI mirror :
doing so prevents the pypi element from fully benefiting image builds,
and the pypi-openstack element can select the OpenStack PyPI mirror
when that is needed.

Change-Id: Ibc73d11d776c89cf1b4ff12cdcd5b8d8b76a176d