gnt-network CLI improvements

- Use unpacking instead of addressing indivdual list items
- Add “--submit” and “--priority” support where possible
- Re-use LUXI client where possible
- Fixed some typos

There is still an issue that a node group named “all” can not be
connected or disconnected. More changes are needed for that.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>