Document complete cluster recovery procedure (escluster)

[cbaxley]

Part of: #737 (Eng Spec: Add clustering support for Elasticsearch)

Description
As a system administrator, I want documentation on recovering from complete cluster loss so that I can restore service from backups.

Acceptance criteria

• Documentation covers deploying fresh ES cluster
• Documentation covers restoring config/secrets from Ansible backup
• Documentation covers restoring data from ES snapshot
• Documentation covers re-enrolling agents after recovery
• Runbook with step-by-step commands is provided