Config file should be usable by firecracker-local and runtime during tests

#346 adds configuration loading the `firecracker-local` package. The problem with this change is the `runtime` and `firecracker-local` have two separate configs, but need one of the values, `shim_base_dir` to be consistent between the files. This is really fragile and is really confusing. Ideally we would write a single config that is used by `firecracker-local` and the `runtime`.