Use structured logs (Python & Go)

#### Description

Logs originating from Cortex (both from the operator and from APIs) should be structured json logs (with fields for log level, etc) rather than strings.

[uber-go/zap](https://github.com/uber-go/zap) seems like a good choice in Go.

May be related to https://github.com/cortexlabs/cortex/issues/1563.

Make sure logs are deleted relatively quickly