Made HTTP check tag aware

The http_check wasn't tag aware. This is problematic when checking multiple virtual hosts on the server that the agent is running on. In such a scenario all of the URLs would be http://localhost and only the "Host" header would be changed; They would all be tagged as url:http://localhost without an ability to add extra tags.

In a scenario where there might be multiple web servers behind a load balancer it's important to be able to check each web server's health instead of referring to the website's URL which would be a VIP shared by the cluster.