Change SecurityImpact destination address.

Change the destination address for SecurityImpact email notifications to
openstack-security@lists.openstack.org. This is being done at the
request of bdpayne.

Change-Id: If548c9ad74bb5e4f53f42ac737150a32ff54006e
Reviewed-on: https://review.openstack.org/27304
Reviewed-by: bdpayne <bdpayne@acm.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Spencer Krum <krum.spencer@gmail.com>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins