Add global variables for the headnode, maint. node etc

[bgruening]

Do we have a more global variable for the headnode, instead of sn09.galaxyproject.eu

Originally posted by @bgruening in #1610 (comment)

[kysrpex]

One option is: after the dual headnode setup there would be an Ansible group for headnodes. One could check if the host belongs to the headnodes group.