Private image repo support

[richcar58]

There are two private image scenarios that we might want to support. The first is when a user makes an image private in the default repository (e.g., Dockerhub); the second is when a user has public or private images in their own repository. In both cases, we should allow users to store their repo credentials in Tapis (SK) and provide a way for them to be used when Jobs tries to launch an image. In the second case, we will probably assume the user has set up their repo properly and reflected it in their application's image parameter.

Currently, the same effect can be achieved using a script like the one suggested in Docker documentation and a job's cmdPrefix parameter.