Summary: Prevent deletion of wrong iptables rules

Detail: A grep in security_group.py wasn't defined well enough, could
potentially delete rules for VMs other than intended

BUG-ID: CLOUDSTACK-309
Bugfix-for: master
Reviewed-by:
Reported-by: Francois Scala
Signed-off-by: John Kinsella <jlk@stratosec.co> 1363222521 -0700