Keystone access from Horizon fails in full_ha

In full_ha horizon is currently configured to access keystone via
localhost, but keystone is configured not to bind to localhost. As
a result, authentication to horizon fails.

Add a parameter to full_ha scenario specifying the location of
keystone for horizon to use.

Change-Id: I1333069f7f6994908deae50b78ae8252593502d8
Closes-Bug: #1273934