CLOUDSTACK-7605: Fix basic zone multiple pod network restart with cleanup=true

1. getRouters() doesn't handle RestartNetwork with cleanup=true for basic zone,
because pod wouldn't be specific at the time.

2. The regression caused by the following fix. The variable "routers" was
overrided with some local values, result in only one of the routers in multiple
pods would return, thus only one router would be started.

commit 6dd5c3fd42c70855d75156243dddc4933436baaf
Author: Rohit Yadav <bhaisaab@apache.org>
Date: Thu Oct 11 18:30:00 2012 +0530
CLOUDSTACK-70: Improve restart network behaviour for basic network