Fix dependency to purge default vhost config
Fix dependency to purge default vhost config

It turned out defining dependency for openstacklib::wsgi::apache
doesn't properly enforce resource order and the default vhost file
is not purged properly.
This change adds the more explicit dependency to enforce the order
properly.

Change-Id: Ifb8df042e06a275ca4dd44342d3c32af65f7d209

openstack/puppet-gnocchi,openstack/puppet-gnocchi