Add an option to set config

[arajasek]

A solution for #3674 that seems to work.

[magik6k]

It doesn't seem to actually be used, repo.Config() will still get you the one in lotus dir

func (fsr *fsLockedRepo) Config() (interface{}, error) {
	fsr.configLk.Lock()
	defer fsr.configLk.Unlock()

	return fsr.loadConfigFromDisk()
}

func (fsr *fsLockedRepo) loadConfigFromDisk() (interface{}, error) {
	return config.FromFile(fsr.join(fsConfig), defConfForType(fsr.repoType))
}

[arajasek]

@magik6k I'm actually overwriting the config in the lotus dir, so the provided one will be used (and persisted in the lotus dir)

[arajasek]

(willing to be told that's terrible, though, it was just the easiest to keysmash)