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".
networkd: remove double-restart workaround

The kernel fixed the underlying issue in 763e0ec and e721f87.
Merge pull request #257 from crawford/networkd

networkd: remove double-restart workaround
Merge pull request #256 from crawford/hyphen

config: deprecate - in favor of _ for key names
script: move Script into config package
config: seperate AssertValid and AssertStructValid

Added an error structure to make it possible to get the specifics of the failure.
config: remove config validator
config/validate: add new config validator

This validator is still experimental and is going to need new rules in the
future. This lays out the general framework.
flags: add validate flag

This will allow the user to run a standalone validation.