Ability to flag projects to use non-standard libraries

devstack and requirements repo are being updated in the
following changes to allow projects to specify a soft
dependency for additional libraries not specified in
global-requirements. In this review, we enable a project
to specify the flag in a project/job's yml file and
pass the flag along down to devstack

Depends-On: I4aa606514131b5dde67d87f5c8db5a3f3e50fc03
Depends-On: I1f195ef9ff1509659848e14ec9936ff6f66a6496

Change-Id: I32a2cd1245a6fad373870362d3399cf1397aa7f1
Merge "Ability to flag projects to use non-standard libraries"