g5k: add "exotic" job type automatically
The "exotic" job type exists to account for users that simply call oarsub -I
without constraints: they will get machines from any available cluster, except exotic clusters.
But in Enoslib, the user always has to explicitly specify a cluster, so there's no added value to this job type. We should add it automatically.
Simple way: always set the "exotic" job type (I will ask the G5K staff if this is OK)
Alternative: lookup the "exotic" field in the API description of the cluster and add it to the job type if one of the requested clusters is exotic.