Configure horizon and horizon URLs in Tempest

Tempest now supports testing the dashboard. Adjust the configuration
accordingly.

Change-Id: Ifb5619caab8bd6080c5df4ed43e16c921e7f9b1f
Added libffi-dev to the needed packages

Change-Id: Ida7001886f17c9413b33312f12f2478a54bd6c90
Merge "Configure horizon and horizon URLs in Tempest"