HA plan for managed MongoDB

[cnelson]

In order to reliably store billing information I want our k8s-brokered MongoDB service to have a multi-az HA plan available.

Acceptance Critera

• [ ] An HA plan exists in production for MongoDB
  • [ ] The plan is multi-az
  • [ ] The plan can auto recover if a single member is lost
  • [ ] The plan backs up data off-system (to an S3 bucket?)
• [ ] A tested and documented plan for how to restore from a backup in case of multi-member failure exists

cc: https://github.com/18F/cg-atlas/issues/155