Update Ubuntu horizon installation instruction
Update Ubuntu horizon installation instruction

openstack-dashboard.conf provided by Ubuntu package does not
contain WSGIApplicationGroup configuration which is required to
make the dashboard work correctly.

Change-Id: I578217571e7a80c174bfc65317d4a1c159ff8692
Closes-Bug: #1738379

ChameleonCloud/horizon,openstack/horizon,noironetworks/horizon,NeCTAR-RC/horizon,openstack/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,noironetworks/horizon,noironetworks/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,openstack/horizon,noironetworks/horizon