Gridmix should require a proxy user's file for impersonating various jobs. So, implement couple of methods for getting the proxy users list and a proxy users file (it's a combination of proxy users and groups) based on cluster configuration.
The proxy users list should require for map reduce jobs and proxy users file should require for gridmix jobs.
The following are methods signature,
public ProxyUserDefinitions getHadoopProxyUsers() - get the list of proxy users list based on cluster configuration.