Add repo_build pip cache

The repo_build role is not using the cache provided by the OpenStack
infrastructure team, make it use it.

Depends-On: I2e42ef4022aaaa2e1cb9f461e63ce2e9e1f49511
Depends-On: I3df2c670beeb78baaa1515bcd27e8f2b0d95b3a9
Change-Id: I0bec4400ebfcb4bf472c58554ee19525be658a40

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

@@ -36,6 +36,7 @@
         if [[ ${NODEPOOL_PYPI_MIRROR} == *.ovh.* ]]; then
           echo "nova_virt_type=qemu"
         fi
+        echo "repo_build_pip_default_index=${NODEPOOL_PYPI_MIRROR}"
       args:
         executable: /bin/bash
       register: nodepool_variables