Allow override next-server for the duration of reservarion #339
Description
In #254 the next-server override was added as admin option. That's fine for machines permanently reserved for a specific use.
However, if users are to be able to test their own medium builds in Orthos this needs to be tied to machine reservation.
Without that, the user needs to ask an administrator to set the option, and remember to ask an administrator to unset it again when they are done with the testing, otherwise the machine will be broken when the next user reserves it.