use master upper-constraints when building tempest venv

The local requirements repo can be checked out to a stable branch,
in which case, the requirements might conflict with tempest's master
requirements.

Master branch's upper-constraints should be used when building tempest's
venv.

Closes-Bug: #1706009

Change-Id: Ifd64638cae2886671421149dbbff3a57f9c64257
1 file changed