use disable/enable_apache_site on Ubuntu

Using the specific Apache2 tools a2ensite and a2dissite
(used in the methods disable/enable_apache_site in lib/apache)
should be the preferred way to disable the default site and
to enable the horizon site.

fixes bug #1298031

Change-Id: I2f247c0b6e605a330086a558e03e2c36666c55be
Merge "use disable/enable_apache_site on Ubuntu"
Merge branch 'master' of github.com:openstack-dev/devstack