Add request.environ[reseller_request] = True if request is coming from an user in .reseller_admin group

I added this to openstack/swift for tempauth & keystone already, this is the patch for swauth. It sets request.environ[reseller_request] = True if the request is coming from an user in .reseller_admin group which can be used by other middlewares. Currently the account_quota middleware makes use of this.