gnt-* {add,list,remove}-tags: Unify options

- Listing tags is a query, so neither “--priority” nor “--submit” make
  sense
- Support both options for adding/removing tags
- Also remove “--submit” from “gnt-node health”; it doesn't work and
  doesn't make sense for listing node health

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>