Allow use custom uppper constraints in stable/stein

Openstack Stein still supports Python 2.7 which means it is necessary to
set different upper constraints for that specific release for tempest
testing using that Python version.
The current master upper constraints won't work as they exclusively
consider Python version >= 3.x

Change-Id: Ieafd0ebc242eba2f4cbaf22823dbe94735236189
2 files changed