Clone grenade for multinode linuxbridge job

The PROJECTS envvar was not updated, so grenade tree was not really
cloned, resulting in job failure.

Change-Id: Iae69f456d790a4d7a29ffbfac71726b860e0fb37

jenkins/jobs/devstack-gate.yaml

@@ -2242,6 +2242,7 @@
           export DEVSTACK_GATE_NEUTRON=1
           export DEVSTACK_GATE_CONFIGDRIVE=0
           export DEVSTACK_GATE_GRENADE=pullup
+          export PROJECTS="openstack-dev/grenade $PROJECTS"
           export DEVSTACK_LOCAL_CONFIG="Q_AGENT=linuxbridge"$'\n'"PHYSICAL_NETWORK=default"$'\n'"LB_PHYSICAL_INTERFACE=eth0"$'\n'
           export BRANCH_OVERRIDE={branch-override}
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then