Add loadbalancer management for staging, production

- Add README note about new Python module dependency
- Add new vars, pre and post tasks for dbnode role
- Add pre and post tasks to Elasticsearch playbook
- Add loadbalancer tasks to API playbook
- Add loadbalancer tasks for site proxy
  Also move siteproxy plays to their own playbook.

Plus improvements to Elasticsearch restart behavior:
- Set Elasticsearch unicast hosts array
  This seems to help avoid split-brain syndrome.
  It also makes node discovery a lot faster.
- Add shard replcation toggling for Elasticsearch
  Before Elasticsearch nodes are restarted, and before we do
  anything that might cause one of them to fail to come back
  up, we disable shard reallocation to keep the cluster from
  transferring huge shard files when it thinks one of the nodes
  has gone down.