Change default config handling

[karlfloersch]

Is your feature request related to a problem? Please describe.
Currently some defaults in the config are set here:
https://github.com/plasma-group/plasma-chain-operator/blob/master/src/utils.js#L148-L163

Describe the solution you'd like
This should be replaced with 1) object.assign, and 2) a "default" config file which exists in an actual file somewhere in the repo, similar to https://github.com/plasma-group/plasma-chain-operator/blob/master/test/config-test.json