allow for NON_STANDARD_REQS override

there are non integrated projects in openstack that use devstack
framework for running jobs. They need a lever in their jobs to
tell the infrastructure that we won't enforce global-requirements
strict compliance on them. We'll still generate a warning so that
they know they are using requirements outside of OpenStack's
accepted list, but it will be non fatal.

Change-Id: Id90547e2fb0a5736385c9f31ba386e7c2e0616df
Merge "allow for NON_STANDARD_REQS override"