Add ability for HttpMonitor to use node label as virtual host for HTTP polls

To be able to successfully poll many HTTP servers, the correct virtual host must be provided. Right now we have the ability to manually specify the virtual hostname on a per-service basis, but a more flexible solution in some cases would be to use the node label of the node as the virtual host. In OpenNMS installations where the nodelabel is set to the DNS name across multiple nodes, this should provide a valid value for the HTTP virtual host without having to define a separate HTTP service for each node.