Default Service or App Context for Topologies

In order to help reduce the complexity and level of granularity of the UI related rewrite rules, it would be good to be able to have UIs deployed to their own topology that has implied context paths. For instance, when authoring the rewrite rules for something like Ambari, we need to add the "ambari" path element in front of every relative path.

Coupled with the need to do this and the need to disambiguate a relative url like images/log.gif between one app another, we end up with a rewrite rule for every single link in a UI.

So, if we can limit UIs to their own topology then we shouldn't have to add "ambari" in front of each link in rewrite rules. But in order to match a topology path in the GatewayFilter we will need to know the missing path element.

This JIRA is an attempt at coding this into the topology.