Added serial option to site.yml.

[OpenStack Infra]

https://review.openstack.org/367264
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/kolla" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 4c1482b999dcea0fa31dac721f97928bbe1796e0
Author: Mick Thompson <email address hidden>
Date: Thu Sep 8 11:07:22 2016 +0100

    Added serial option to site.yml.

    Useful for upgrade etc., which is preferablly done serially.

    Example usage: tools/kolla-ansible deploy OR tools/kolla-ansible upgrade

    Closes-Bug: #1576708

    DocImpact

    Change-Id: I34b2e16f8ce53e472a4682a4738c4ac0f5abf00c