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

[xibz]

#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.