allow for soft updating of global-requirements

This creates a devstack REQUIREMENTS_MODE which is how we handle
syncing of global requirements. The default is 'strict', which is
current behavior. There is a new 'soft' mode which does a
--soft-update for projects *not* found in projects.txt, which lets
them specify additional requirements.

Change-Id: I4aa606514131b5dde67d87f5c8db5a3f3e50fc03
Depends-On: I1f195ef9ff1509659848e14ec9936ff6f66a6496
Merge "allow for soft updating of global-requirements"