g5k: allow to specify different env_name for different machines
Currently, env_name is a global parameter, which means we deploy the same environment to all machines.
It would be useful to be able to deploy a different environment on different groups of machines: e.g. a control node with Debian and compute nodes with Ubuntu.
It should not be very hard to implement, we just need to group machines according to site+env_name when calling kadeploy. Deployments should be parallelized if possible (but there's a limit to the number of concurrent deployments we can start on a given G5K site, so this needs to be tested).
This is low-priority, I don't have actual use-cases for this yet.