Fix scope of vhost template variables

Dynamic scoping for variables in ERB templates was removed in puppet
4[1] which means that the variables defined in the
openstack_health::vhost class cannot be found when it is referenced in
the httpd::vhost defined type and will be evaluated as nil when
puppet runs. Use scope.lookupvar instead to be explicit about the
variable's source.

[1] https://puppet.com/docs/puppet/4.10/lang_updating_manifests.html#dynamic-scoping-in-erb

Depends-On: https://review.openstack.org/527302

Change-Id: I19a55d079b236728f1311e130ec6ea851539b446

templates/openstack-health-frontend.vhost.erb

@@ -9,7 +9,7 @@ Listen <%= @vhost_port %>
   CustomLog ${APACHE_LOG_DIR}/openstack-health-frontend-access.log combined
   ErrorLog ${APACHE_LOG_DIR}/openstack-health-frontend-error.log
 
-  <Directory <%= @frontend_dir %>>
+  <Directory <%= scope.lookupvar('::openstack_health::vhost::frontend_dir') %>>
     Order allow,deny
     Allow from all
     <IfVersion >= 2.4>
@@ -17,6 +17,6 @@ Listen <%= @vhost_port %>
     </IfVersion>
   </Directory>
 
-  DocumentRoot <%= @frontend_dir %>
+  DocumentRoot <%= scope.lookupvar('::openstack_health::vhost::frontend_dir') %>
   DirectoryIndex index.html
 </VirtualHost>