don't UNDO_REQUIREMENTS after we update from g-r

it turns out there was a loophole in the global-requirements to
rule them all approach, because tempest runs under tox, and the
default devstack behavior is to reset the source trees after
global-requirements was used for setup.py develop, so that we
wouldn't be poluting people's working trees.

This meant that if tempest contained an incompatible requirement
from g-r, it could wedge the gate (which it did for stable/havana).

We've since introduced UNDO_REQUIREMENTS in devstack that lets us
control this reset. This change in the gate will ensure that we
don't reset tempest back to a state where it can wedge us.

Change-Id: I6fb14cef9d52a35569c2f0215267609cd6741337

devstack-vm-gate.sh

@@ -111,6 +111,8 @@ VIRT_DRIVER=$DEVSTACK_GATE_VIRT_DRIVER
 SWIFT_REPLICAS=1
 LOG_COLOR=False
 PIP_USE_MIRRORS=False
+# Don't reset the requirements.txt files after g-r updates
+UNDO_REQUIREMENTS=False
 CINDER_PERIODIC_INTERVAL=10
 export OS_NO_CACHE=True
 EOF