Feature/extra groups on nibbler

[pneerincx]

• Nibbler group vars: commented source (dh1 lustre) for pfs 'umcgst01' to prevent mounting it already, which would be prematurely. This file is not ready yet for use with Nibbler.
• Bugfix: removed non-admin groups and users from list of admin groups and users on Marvin.
• Bugfix: Replaced several hard coded group and user names with variables where possible for several clusters.
• Bugfix: Added missing groups and users on Nibbler.
• Bugfix: Added missing gid for funad group in group_vars/all/vars.yml.
• Bugfix: Improved creation of group folders on local volumes. Use dedicated variable to indicate which group folders should be created on which local volumes on which machines as opposed to simply using the list of all regular groups, which would create the required folders, but may also create unnecessary/unwanted folders in other places.