Add label to specify whether to mount volumes or not

Adds a new label, toolforge.org/mount-storage, to specify whether to
mount nfs volumes or not. By default all volumes are mounted, but the
label can be set to 'none' to not mount anything.

Bug: T346605

Related MRs:

• !6 (merged) (This MR)
• !5 (merged)
• !4 (merged)
• !3 (merged)
• main