Cluster data can be backed up

[adamhosier]

Cluster owners should be able to set a backup strategy on their etcd clusters in order to take automated backups of cluster data. Backups must be able to be taken on demand, and on an automated schedule. We must implement at least one backup strategy suitable for use in Improbable's infrastructure - likely dumping of data to some cloud storage bucket.

Acceptance criteria:

• The API is extended with capability for performing backups
• A backup strategy for dumping cluster data to a GCS bucket (or more generic) is implemented
• An e2e test exists which requests a backup, and observes that one is created.
• Unit tests exist for automated backups (using something like https://github.com/jonboulle/clockwork)
• README is updated to reflect that backups are functional