Perform additional disable_service checks

With the advent of plugins and their settings files it has become
possible to disable_service in local.conf only to have the service
re-enabled in a plugin settings file. This happens because of
processing order.

To get around this the disable_service function now aggregates
service names into a DISABLED_SERVICES variable which is then checked
during enable_service. If something tries to enable something that
was previously disabled, a warning is produced in the log and the
service is not enabled.

Then after all configuration has been sourced a final check is to
done by verify_disabled_services to confirm that something has not
manually adjusted ENABLED_SERVICES to overcome a previously called
disable_service. If something has, the stack dies with an error.

Change-Id: I0f9403f44ed2fe693a46cd02486bd94043ce6b1a
Closes-Bug: #1504304
diff --git a/stack.sh b/stack.sh
index db0ff98..b65c558 100755
--- a/stack.sh
+++ b/stack.sh
@@ -553,6 +553,7 @@
 # Phase: source
 run_phase source
 
+
 # Interactive Configuration
 # -------------------------