Submission v2 init-container should pull remote dependencies as well as submitter-local ones

[mccheah]

In #227 we created an init-container that only runs when the application submitter is providing local dependencies through the resource staging server. However, the init-container also needs to fetch remote application dependencies that were provided to spark-submit. We need these dependencies so that the classes are available to the driver application before the application actually starts. A simple example is the user's main class being hosted in a jar stored in HDFS; right now the Spark driver will try to launch with a main class that doesn't exist.

[mccheah]

I think it might be neater to have a separate init-container for fetching remote things. This creates a nice architecture for the submission client where configuration of the init container doesn't have to be split between the Client and the MountedDependencyManager. Currently the MountedDependencyManager has a lot of specific logic for configuring how its init-container should fetch dependencies from the resource staging server specifically (e.g. TLS), but we would need to inject extra configuration into that init-container if we wanted that to be able to handle arbitrary URIs as well. Instead we could just chain two completely separate init-containers together and MountedDependencyManager and its concerns can remain as a completely separate/isolated component.

[mccheah]

On second thought this would require creating separate docker images for each init container, so perhaps the overhead and the configuration complexity of needing to have too many docker images just to start a driver might not be worth it.

[mccheah]

This is done.