disable errexit for the test run phase of the script

we'd like to be able to control things with more granularity
towards the end of runs, not just blow up on errexit. So disable
this for the end of the runs before we go into tempest.

Change-Id: Ibe0e56f3ffd7d084d549562bcfd0069b32fef6df

devstack-vm-gate.sh

@@ -311,6 +311,9 @@ if [ "$DEVSTACK_GATE_TEMPEST" -eq "1" ]; then
         sudo chmod -R o+rx /opt/stack/new/devstack/files
     fi
 
+    # let us control if we die or not
+    set +o errexit
+
     cd $BASE/new/tempest
     if [[ "$DEVSTACK_GATE_TEMPEST_ALL" -eq "1" ]]; then
         echo "Running tempest all test suite"