config/update: add "off" as a valid strategy

It was assumed that the user would specify the reboot strategy as an
unquoted value. In the case that they turn off updates, `off` is
interpreted as a boolean and the normalization pass converts that to
`false`. In the event that the user uses `"off"`, it's interpreted as a
string and not modified.
Merge pull request #265 from crawford/update

config/update: add "off" as a valid strategy