Fix startup of apache and sni proxy node frontend plugins

Also enable configuration of apache-vhost and haproxy-sni-proxy node
frontends as well, previously the options and classes where there, but
they were never invoked.

Update docs to indicate that apache-vhost will be ignored as a frontend
plugin if apache-mod-rewrite is also selected
Merge pull request #172 from detiber/FixNodePluginStart

Fix startup of apache and sni proxy node frontend plugins