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