Exit when printing error about database config

When finding no configured database, devstack prints
a suitable error in the logs

   "No database enabled"

but then just carries on running, as a result the developer
is very unlikely to ever see the original error message.
Change the 'echo' to a 'die' so that it stop immediately
making the error message clearly visible.

Change-Id: Ibd86bfcb5d4a3b90a1ee7a5bd637b01124e3a6ba
diff --git a/stack.sh b/stack.sh
index 4602b0b..b03cca8 100755
--- a/stack.sh
+++ b/stack.sh
@@ -606,7 +606,7 @@
 # The available database backends are listed in ``DATABASE_BACKENDS`` after
 # ``lib/database`` is sourced. ``mysql`` is the default.
 
-initialize_database_backends && echo "Using $DATABASE_TYPE database backend" || echo "No database enabled"
+initialize_database_backends && echo "Using $DATABASE_TYPE database backend" || die $LINENO "No database enabled"
 
 
 # Queue Configuration