Drop the -g shortcut for --vg-name

Changing the volume group is a lot less frequent than acting on a node
group. As such we drop the "-g" shortcut and require the long option to
be passed. In 2.3 the commands which used to accept the volume group as
"-g" won't have any node group option, so no confusion will arise. Later
on we may pass "-g" as the initial node group name to gnt-cluster init,
although that's not strictly necessary, as modifying it later is always
possible.

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