Allow cloning clusters from the operator. (#90)

Allow cloning clusters from the operator.

The changes add a new JSON node `clone` with possible values `cluster`
and `timestamp`. `cluster` is mandatory, and setting a non-empty
`timestamp` triggers wal-e point in time recovery. Spilo and Patroni do
the whole heavy-lifting, the operator just defines certain variables and
gathers some data about how to connect to the host to clone or the
target S3 bucket.

As a minor change, set the image pull policy to IfNotPresent instead
of Always to simplify local testing.

Change the default replication username to standby.