The `file` provider was not considering the global defaults for
`MaxTags`, `IncludeTags`, and `ExcludeTags`.
Updated unit tests to validate they are now considered.
Allows setting of image configs at a global level to act as default
values.
This required a change in the model.Image struct due to a bool field not
having a third, unset state. The remedy is to unmarshal into a temporary
data structure to detect the presents of a field value and then use that
to determine if the default value should be used.
Fixes#491
