Install gnocchi from source

... so that we install the expected stable branch. Without this
the latest version available in pypi is always used.

Change-Id: I2b121acc70e5deb86b00e2739d9129e737985e7e
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

.zuul.yaml

@@ -20,7 +20,8 @@
       - openstack/heat
       - openstack/diskimage-builder
       - openstack/tempest
-      - gnocchixyz/gnocchi
+      - name: gnocchixyz/gnocchi
+        override-checkout: stable/4.7
       - openstack-k8s-operators/sg-core
     vars: &base_vars
       configure_swap_size: 8192
@@ -37,6 +38,7 @@
         tempest: true
         node_exporter: false
       devstack_localrc:
+        LIBS_FROM_GIT: gnocchi
         TEMPEST_PLUGINS: '"/opt/stack/telemetry-tempest-plugin /opt/stack/heat-tempest-plugin"'
         GNOCCHI_ARCHIVE_POLICY_TEMPEST: "ceilometer-high-rate"
         CEILOMETER_BACKENDS: "gnocchi,sg-core"