Add support to resolve artifacts from private docker registry

[sabbyanandan]

As a user, I'd like to resolve artifacts from a private Docker registry.

Notes:

• similar to k8s-deployer, we will have to add support for imagePullSecret
• see this SO thread for more details

[ericbottard]

Given that The local deployer uses the regular docker command, I believe there is nothing that can/should be done for this to work.
If the user has successfully authenticated against said repository (using docker login my.jfrog.io) then everything should just work.

Background reading: https://docs.docker.com/registry/deploying/

[ericbottard]

Note that the format of authentication for the particular repository may be a bit peculiar, which may explain issues the original user may have.
For example, reading the doc about bintray: https://bintray.com/docs/usermanual/formats/formats_dockerrepositories.html

But again, I believe there is nothing we have to do at the SCDF level. All matters of authentication (even k8s pullSecret, which have nothing to do here) require that authentication is first done locally in docker. Once this is done, then things should "just work" given we're using the docker client