Configure haproxy keystone backend before keystone service

Configure haproxy keystone backend before keystone service
starts. This will fix ordering problem when we have keystone
resources created before keystone is available. Service
is already started before resources creation, so that
puppet provider will receive a couple of 503|504 HTTP
errors in the worst case and handle them.

Closes-bug: #1357386

Change-Id: I906fb01f678b0407e421a5db7f617901fed80f79