Enabled the Horizon vhost priority to be set.

Previously, the apache vhost priority was hard coded to '15' in
puppet-horizon. This does not allow the vhost priority to be
managed, even with $extra_params, due to apache looking for either
default_vhost OR priority. Defaults to '10' to make Horizon the
default vhost.

Change-Id: I95fe08b411eaaf668d707cb6ccd7e118dfe0a9ed
Depends-on: #91461
Merge branch 'master' of https://github.com/stackforge/puppet_openstack_builder into icehouse