Adding params to run different agents using different users

I need to run each agent in a different user (in order to keep different maven local repo + kill some processes before the build). 

So this PR allow us to redefine the user for each of the agents. It should _not_ break any previous behaviour. 
The reason why it's not managed is because it would be much more complicated to guarantee one User resource per module. And I can create it outside, so it's good enough to go.