Fix pip invocation for old versions of pip.
Fix pip invocation for old versions of pip.

Fixes bug 1190749.

Change-Id: I91fce8ee1c37f945062e81c1585ee45613e05371

citrix-openstack-build/pbr,WiFast/pbr,redhat-openstack/pbr,openstack-dev/pbr,varunarya10/pbr,openstack-dev/pbr,openstack-dev/pbr,citrix-openstack-build/pbr,citrix-openstack-build/pbr,klmitch/pbr,varunarya10/pbr,klmitch/pbr,WiFast/pbr,redhat-openstack/pbr,JioCloud/pbr,JioCloud/pbr