chown stack/data/nova/networks to $STACK_USER

$USER may be undefined (resulting in an error exit from stack.sh)
or may be the current $USER rather than the target $STACK_USER
(which may or may not be the same) which could lead to errors
later.

This change brings about consistency: the rest of stack.sh always
uses $STACK_USER.

This same problem may be present in lib/marconi.

Change-Id: Idff2bee88de4e1da11cd5765a03646caf3ede24f
diff --git a/stack.sh b/stack.sh
index 8fb57c4..d2f18d4 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1026,7 +1026,7 @@
     if is_service_enabled n-net; then
         rm -rf ${NOVA_STATE_PATH}/networks
         sudo mkdir -p ${NOVA_STATE_PATH}/networks
-        safe_chown -R ${USER} ${NOVA_STATE_PATH}/networks
+        safe_chown -R ${STACK_USER} ${NOVA_STATE_PATH}/networks
     fi
 
     # Force IP forwarding on, just in case