Simplify InitConfig and remove SimpleConfigWriter

InitConfig currently creates the cluster config_data, then puts it into
a dict, passes it to SimpleConfigWriter to load it from a dict (which
just reuses the dict value) and then saves it. The SimpleConfigWriter is
then returned, but ignored. With this patch we just write out the
config_data at InitConfig time, and thus can remove SimpleConfigWriter
altogether. The now unused SimpleConfigReader.FromDict is also gone.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>