Instance attributes usable as {placeholders} in host URL suffixes

In the process of implementing a ZooKeeper instance discovery strategy, I found myself needing to dynamically replace the port Turbine uses to connect on a dynamic basis. This PR implements the ability to replace any instance attribute in the host URL suffix (e.x. {server-port}).

We run multiple workers per host machine (Storm) and embed a Jetty webserver which binds to any available port above a certain value. Given that multiple workers run per machine, the default port can often be used. We register this port in ZooKeeper but had no way to asking Turbine to connect to it.