Fix gnt-instance start/stop/… args

The gnt-instance commands that have the multi-filter can take also zero
arguments, if the --all flag is passed. But they currently are declared
with min=1, and this is a regression due to the recent bash completion
changes.

There is also the problem that --primary/--secondary expect a node and
not an instance, but currently the completion doesn't support this
distinction.

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