Deferring the uploading of VM Resources

[sdelliot]

This is a sub-component of #32 relating to deferring the uploading of VM Resources. In the current structure, images/VM resources are uploaded as MCs are executed during the experiment in dependency-graph order. This issue proposes creating a new model component (likely in firewheel_repo_base) which will iterate through all prior model components and upload their resources prior to instantiating the VMs with minimega. There are some notable challenges:

1. MCs are currently unaware of the entire dependency graph. The simplest way to resolve this issue would be to pass in the ordered list of model components to each model component via the AbstractPlugin.
2. A new MC would need to be created and it's release would need to be coordinated with the API change.
3. Assumptions about availability of VM resources could be impacted if MCs provide VMRs or images and are executed after the new uploading MC. This could be mitigated if the uploading MC uploads all images/VMRs in the experiment graph, but the side effects are unexplored.

An alternate possibility is to optionally keep the existing auto-upload functionality but provide an option in the build_experiment_graph->process_model_component process. This could be combined with a new upload MC, or used as a way of having a "better" experiment dry-run that executes code, but doesn't upload images or launch VMs.