makisu pull: add support for --registry-config

[lbpdt]

This is useful to authenticate with the registry when pulling images.

[CLAassistant]

All committers have signed the CLA.

[lbpdt]

Hi @sema, would you be able to review this feature?

Also, it looks like the Fossa check on Travis has been failing for the past couple months, if that's something you can fix.

TROUBLESHOOTING: A FOSSA API key is needed to run this command.

[sema]

Thanks for adding this!

[bburky]

I implemented something similar in #354. It also seemed that deleting the DockerHubRegistry code was needed to allow pulling from docker.io. The rest of makisu only allows configuration with --registry-config and doesn't have hardcoded docker config at all.

It's a little confusing to leave the hardcoded docker.io config if this registry-config option is added, because it cannot be overridden (I think so at least).

[lbpdt]

@bburky yours is very similar indeed, but with a larger scope than what I'm doing here. You could also rebase your PR on top of master if this one is merged first, so your change can be narrowly focussed on updating the hardcoded docker.io registry.

[lbpdt]

Build is now failing due to rate limiting on DockerHub. I'll kick it again is a couple hours.

library/golang
toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit

EDIT: build now passes but the Fossa check is still missing an API key...