When neutron subnet-show is executed with an invalid name/id

When neutron subnet-show is executed with an invalid name/id
it still yields a result.

This is because of a bug in cotrail plugin which does not apply
any filtering on name as subnet name is not supported yet.

This patch is to address that bug.

Signed-off-by: Babu Shanmugam <anbu@enovance.com>