Fix incorrect config-external in horizon

Horizon config-external.sh script has the same source and target.
Corrected the code to operate like Nova does.

Closes-bug: #1478451

Change-Id: I80be730d41e126b8a43c24c5eb87dfe68ffddb48

docker/common/horizon/config-external.sh

 #!/bin/bash
-SOURCE="/etc/openstack-dashboard/local_settings"
+SOURCE="/opt/kolla/horizon/local_settings"
 TARGET="/etc/openstack-dashboard/local_settings"
 OWNER="horizon"