Move horizon apache root to /dashboard

With keystone's move to /identity, a conflict in for resources was
created as both keystone and horizon used /identity. The keystone
config took precedence and rendered API output in the horizon UI.

This patch sets the root for horizon to /dashboard and serves all
horizon content from there. Additionally, a RedirectMatch has been added
to the apache config for horizon to redirect '/' to '/dashboard' this
will allow the implementation to change without being immediately
painful to users.

Also made the path '/dashboard/' configurable in stackrc.

Closes-Bug: #1478306
Depends-On: I9a04f936ed6d8c14775a332dc28e903992806c42

for devstack-gate changes to remove hard coded horizon url structure
assumptions.

Change-Id: I6fbca5cea9e44df160afbccc71bd045437657320
diff --git a/stack.sh b/stack.sh
index cc8bc8c..cfcf126 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1421,7 +1421,7 @@
 # If you installed Horizon on this server you should be able
 # to access the site using your browser.
 if is_service_enabled horizon; then
-    echo "Horizon is now available at http://$SERVICE_HOST/"
+    echo "Horizon is now available at http://$SERVICE_HOST$HORIZON_APACHE_ROOT"
 fi
 
 # If Keystone is present you can point ``nova`` cli to this server