Teleport nodes should start with stale data

[russjones]

Teleport nodes should start even if they can not connect to auth as long as they have cached access to the the backend. By default the cache should be for 24 hours, but we should support making this configurable. We also need to add this information to our documentation (what customers can expect for Teleport, what signals to send so the cache can be closed without currupting it, and how to change cache validity).

[russjones]

Something like this.

cache:
  # Not sure what disk backed/sqlite one is called. Might want
  # to check this.
  type: "sqlite"

  # Default cache validity is 24 hours. This value can be increased
  # or decreased based off tolerance of serving stale data.
  ttl: "24h"

[russjones]

You'll probably also need to add documentation explaining the above and point out what signal the user should send to Teleport so it can safely write the tombstone to disk and not corrupt the cache. @fspmarshall can help you on that as well.