As of now, due to bug 1817555, networking-sfc code cannot be pulled
from the master branch even if we specify it in 'required-projects'.
As a workaround, this commit specifies networking-sfc stein b1
in requirements.txt (and lower-constraints.txt accordingly) like
what was done in https://review.openstack.org/#/c/639363/
This is likely a better short term solution than moving those
requirements into their own file as we did in