(Consistent) support for multiple machine types

[Fra-nk]

Not all of ROCED correctly handles the presence of multiple machine types correctly.

→ Add consistent behavior while still allowing different sites to manage individual information for each machine type (image id, etc.).

[Fra-nk]

High(er) priority issue, because this should solve a few problems in one swoop:

• Different user groups can easily be mapped to different machine types (ams-default, cms-default, atlas-default)
• System specific variations, e.g. different wall-times can be used to requests machines with different parameters (vm-short, vm-medium, vm-long)