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/stackrc b/stackrc
index 8beef96..f4245a4 100644
--- a/stackrc
+++ b/stackrc
@@ -87,6 +87,9 @@
 # Set the default Nova APIs to enable
 NOVA_ENABLED_APIS=ec2,osapi_compute,metadata
 
+# Set the root URL for Horizon
+HORIZON_APACHE_ROOT="/dashboard"
+
 # Whether to use 'dev mode' for screen windows. Dev mode works by
 # stuffing text into the screen windows so that a developer can use
 # ctrl-c, up-arrow, enter to restart the service. Starting services