Start using dogpile caching in devstack tests

As a run up to enabling the dbm dogpile caching in nodepool, enable it
in the devstack tests.

Change-Id: Id3bd1aca930053674af7848f848bb95b7f6ad85a

devstack/plugin.sh

@@ -234,6 +234,10 @@ EOF
     cp /etc/openstack/clouds.yaml /tmp
     cat >>/tmp/clouds.yaml <<EOF
 cache:
+  max_age: 3600
+  class: dogpile.cache.dbm
+  arguments:
+    filename: $HOME/.cache/openstack/shade.dbm
   expiration:
     server: 5
     port: 5