Modify the default output of gnt-instance list

This patch adds a new field available for selection in gnt-instance list
names "status" which represents the combined value of "admin_state" and
"oper_state". Since this is much easier to parse (e.g. gnt-instance list
|grep ERROR), we also modify the default field list to use this instead
of the admin/oper state fields.

Reviewed-by: imsnah