Allow pip installation to be skipped.
Allow pip installation to be skipped.

Distro packagers don't really need for us to attempt to pip install
things for them. They're usually managing that dependency tree
in the distro packaging. Add an env var they can set in their spec
or rules files to skip any pretense of doing anything with install
requirements.

Change-Id: Ia6387eab3267fa3c4019c4a9d90e4780319c2397

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