Fix grenade job

When upgrading from Queens to Rocky,
since neither release has the tempest plugin in-tree,
it has to be explicitly installed
in the job.

Closes-Bug: #1785370
Change-Id: I77b5e0eb8824a8a00633aacb45ebea07572503d5

.zuul.yaml

@@ -65,6 +65,7 @@
       - openstack-infra/devstack-gate
       - openstack/manila
       - openstack/python-manilaclient
+      - openstack/manila-tempest-plugin
 
 - job:
     name: manila-tempest-base

playbooks/legacy/grenade-dsvm-manila/run.yaml

@@ -28,6 +28,7 @@
           set -e
           set -x
           export PROJECTS="openstack-dev/grenade $PROJECTS"
+          export PROJECTS="openstack/manila-tempest-plugin $PROJECTS"
           export DEVSTACK_PROJECT_FROM_GIT="python-manilaclient"
           export PYTHONUNBUFFERED=true
           export DEVSTACK_GATE_NEUTRON=0