Use the infrastructure provided reverse cache for Percona

We see a lot of timeouts with the Percona repositories so hopefully
with this patch we will be hitting the reverse proxy cache which means
less trips to the internet and less gate failures!

Depends-On: I19be46fc3ac7e8938178d9d8cb4d1dfeadc6f00b
Depends-On: I4d9d408f99608f1625dac9e0802e8d8d77b0f48b
Change-Id: Ieab123e42517c643b69e010a638a677c6363835f

common-tasks/test-set-nodepool-vars.yml

@@ -31,6 +31,7 @@
         echo "pip_default_index=${NODEPOOL_PYPI_MIRROR}"
         echo "pip_wheel_mirror=${NODEPOOL_WHEEL_MIRROR}"
         echo "centos_epel_mirror=${NODEPOOL_EPEL_MIRROR}"
+        echo "galera_percona_xtrabackup_repo_host=${NODEPOOL_PERCONA_PROXY}"
         if [[ ${NODEPOOL_PYPI_MIRROR} == *.ovh.* ]]; then
           echo "nova_virt_type=qemu"
         fi