Make use of STACK_USER instead of relying on USER

Quite easily one ends up calling ./stack.sh in an environment that,
albeit being user "stack" (for example), doesn't quite meet the
expectations of devstack.  The errors that follow can be rather hard to
track down, as the dependency on `USER` is not mentioned.

To remedy this situation, this commit

- uses STACK_USER instead of USER and
- mentions that dependency in the script headers of lib/*

Change-Id: If4cdc39b922ea64b4c0893a0e695ec06349fccc5
diff --git a/lib/ceilometer b/lib/ceilometer
index dcadb07..87bb656 100644
--- a/lib/ceilometer
+++ b/lib/ceilometer
@@ -67,10 +67,10 @@
     setup_develop $CEILOMETER_DIR
 
     [ ! -d $CEILOMETER_CONF_DIR ] && sudo mkdir -m 755 -p $CEILOMETER_CONF_DIR
-    sudo chown $USER $CEILOMETER_CONF_DIR
+    sudo chown $STACK_USER $CEILOMETER_CONF_DIR
 
     [ ! -d $CEILOMETER_API_LOG_DIR ] &&  sudo mkdir -m 755 -p $CEILOMETER_API_LOG_DIR
-    sudo chown $USER $CEILOMETER_API_LOG_DIR
+    sudo chown $STACK_USER $CEILOMETER_API_LOG_DIR
 
     iniset_rpc_backend ceilometer $CEILOMETER_CONF DEFAULT