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

doc/source/install/install-ubuntu.rst

@@ -21,9 +21,6 @@ Install and configure components
 
 .. include:: note_configuration_vary_by_distribution.txt
 
-
-
-
 1. Install the packages:
 
    .. code-block:: console
@@ -32,11 +29,6 @@ Install and configure components
 
    .. end
 
-
-
-
-
-
 2. Edit the
    ``/etc/openstack-dashboard/local_settings.py``
    file and complete the following actions:
@@ -177,6 +169,12 @@ Install and configure components
      For more information, see the `list of time zones
      <https://en.wikipedia.org/wiki/List_of_tz_database_time_zones>`__.
 
+3. Add the following line to
+   ``/etc/apache2/conf-available/openstack-dashboard.conf`` if not included.
+
+   .. code-block:: none
+
+      WSGIApplicationGroup %{GLOBAL}
 
 Finalize installation
 ---------------------