comparison to stable/grizzly is not numeric

the current d-g code uses -ne which is an explicitly numeric
opperator. Make this != to do the right thing with strings. Use variable
that is local to setup_localrc().
Without this there we get:

[[: stable/grizzly: division by 0 (error token is "grizzly")

Change-Id: Ie338b33e4bc655cb8e06b8bf80ab75bb58bf0e5e

devstack-vm-gate.sh

@@ -346,7 +346,7 @@ if [ "$DEVSTACK_GATE_TEMPEST" -eq "1" ]; then
         res=$?
     fi
 
-    if [[ "$LOCALRC_BRANCH" -ne "stable/grizzly" ]] && \
+    if [[ "$GRENADE_OLD_BRANCH" != "stable/grizzly" ]] && \
        [[ "$DEVSTACK_GATE_TEMPEST_STRESS" -ne "1" ]] ; then
         tools/check_logs.py -d $BASE/new/screen-logs
         res2=$?