don't setup cells if n-cpu isn't also running

create_cell requires n-api and at least one n-cpu up and running. If
we have a configuration where it is not guarunteed that there is an
n-cpu at the end of a devstack run we have to skip this step and make
the user run it manually later.

Change-Id: I2287ab29f3c1a7252271dcce81673ef365615296
diff --git a/stack.sh b/stack.sh
index 9c63f5f..05a7666 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1386,7 +1386,15 @@
 
 # Do this late because it requires compute hosts to have started
 if is_service_enabled n-api; then
-    create_cell
+    if is_service_enabled n-cpu; then
+        create_cell
+    else
+        # Some CI systems like Hyper-V build the control plane on
+        # Linux, and join in non Linux Computes after setup. This
+        # allows them to delay the processing until after their whole
+        # environment is up.
+        echo_summary "SKIPPING Cell setup because n-cpu is not enabled. You will have to do this manually before you have a working environment."
+    fi
 fi
 
 # Bash completion