BUG: happi container-registry accesses internals rather than public API

[tangkong]

This seems to break happi container-registry for me. I'll keep looking into it though

This was a bug in the CLI code, reaching into the internals instead of using public API. (f60804d)
Apparently this is also uncovered in the test suite?

Originally posted by @klauer in #270 (comment)

With lazy loading of the container registry, this causes the container-registry command to return nothing
This should also be covered in the test suite