Allow running nova compute with docker group

A use case is from Zun project that needs to start n-cpu with
"docker" group.

Change-Id: Ib8f193ea1edf1f148e9ba505205495170ebf6d67
diff --git a/stackrc b/stackrc
index c419ef4..bf92a77 100644
--- a/stackrc
+++ b/stackrc
@@ -589,6 +589,9 @@
     lxd)
         LXD_GROUP=${LXD_GROUP:-"lxd"}
         ;;
+    docker)
+        DOCKER_GROUP=${DOCKER_GROUP:-"docker"}
+        ;;
     fake)
         NUMBER_FAKE_NOVA_COMPUTE=${NUMBER_FAKE_NOVA_COMPUTE:-1}
         ;;