modify rake clean for ccng v2

filter system domain, since system domain cannot be removed

Change-Id: I33fab5addbe36dc7fc95ef4e32429790a8f6031a
Signed-off-by: jgong <jgong@rbcon.com>
Signed-off-by: Pin Xie <pxie@vmware.com>
Reviewed-on: http://reviews.cloudfoundry.org/13322
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Michael Zhang <zhangcheng@rbcon.com>