config: deprecate - in favor of _ for key names

In all of the YAML tags, - has been replaced with _. normalizeConfig() and
normalizeKeys() have also been added to perform the normalization of the input
cloud-config.

As part of the normalization process, falsey values are converted to "false".
The "off" update strategy is no exception and as a result, the "off" update
strategy has been changed to "false".