Reworking logs

[vishalbollu]

closes #1732

• reduce AWS specific logging implementation in favour generic structured logging tagged with cortex specific labels
• update cortex application code to use json structured logging
• use fluent-bit to massage and stream logs to a log aggregation tool, (Cloudwatch for AWS and Stackdriver for GCP)
• update cortex logs CLI command to stream logs from a single running worker/replica as necessitated by the removal of AWS specific logging implementation

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checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)
• update examples
• update docs and add any new files to summary.md (view in gitbook after merging)
• cherry-pick into release branches if applicable
• alert the dev team if the dev environment changed

[miguelvr]

Just some minor nit's and some questions regarding logging, but overall LGTM