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. 
Instead, I could have added this to agents_default. But it doesn't sound right, as it would be a little bit harder to understand. Let me know if you think it should be different.