Unused docker image retention policy

## Motivation

-

## Objective

- Make agent periodically clean up images which have not been used for given amount of time

## Expected Sub Issue

- Distinguishing images that are meant for [Backend.AI](http://Backend.AI) kernels and are not
- New config key to declare retention period for kernel images
- New agent RPC function which removes image from local container agent
- New manager timer to scan images to be cleared based on the retention timeout

JIRA Issue: BA-3037