Restore previously inapplicable sentence from ASF.

* modules/openstack_project/files/gerrit/cla.html: At the request of
Jonathan Bryce, Executive Director for the OpenStack Foundation,
this change updates the OpenStack Project Individual Contributor
License Agreement with a "public benefit" statement taken from the
Apache Software Foundation's ICLA. This sentence was previously
inapplicable under OpenStack LLC but can now be restored under the
OpenStack Foundation.

Change-Id: I944e1d361af8f0faaed0dc88ded216451b72e4b3
Reviewed-on: https://review.openstack.org/22396
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins