Use REQUIREMENTS_MODE switch and remove pip install

Change id I32a2cd1245a6fad373870362d3399cf1397aa7f1 introduces
a way for projects to depend on additional python libraries not
mentioned in the global requirements

Change-Id: I3a04a453ca3551e280de125cfd548593f12f94f7
Merge "Use REQUIREMENTS_MODE switch and remove pip install"