Add support for clustered deployments

[dfarrell07]

A long-term goal of ODL's deployment efforts has been to support standing up ODL clusters. The Puppet module would have needed a Puppet Master to reasonably handle a multi-machine deploymnet, which would be an undesirable complexity bump for our users. The drive to build an ODL Ansible role was largely spurred by the need to support clustering use-cases and Puppet's inadequacy for doing so.

This issue will track progress towards clustering in general. Additional ones will be raised as-needed for specific, more narrowly scoped, issues.

Main ODL clustering docs.

[dfarrell07]

This is being tracked on an upstream Trello card.