Relax security group rules cleanup

Security groups rules are automatically deleted when we delete the
security group so this is not really necessary, besides that,
nova-network doesn't provide a way to get current status of a rule
(a proof of concept here https://review.openstack.org/#/c/85725/) which
means that we never know if it is really deleted.
Currently it works cause we don't really check the timeouts, but a the
next patch of this series - https://review.openstack.org/#/c/82709/ will
and depends on this to work.

Change-Id: If82d1727b2c5f752e48a2da2c8a029e346f3593e
Merge "Relax security group rules cleanup"