spacemanager: Add not null and unique constraints to link group name

Since link group name is the only identifier exposed throgh
the UI, this patch marks the field as not null. This requirement
isn't new as the name has always been the only synchronization
point between pool manager and space manager.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6469/