Updates support for Ubuntu to install Horizon.
Update the Horizon install block to follow the same structure as all
of the other source install blocks.
Removed local_settings copy since that will be copied in from outside
with the config-external.sh script
Change-Id: Iab386386286d66e6f74407a043e78cb5cb1b032b
Partially-Implements: blueprint install-from-ubuntu
6a297265c5