COOK-4266 Unless explicitly set, gpgcheck should be true only if gpgkey is set

https://tickets.opscode.com/browse/COOK-4266


With yum >=3.0.4, the default value for gpgkey is true.
It used to be true only if a gpgkey was set.
This leads to invalid configuration for repositories with no signed packages.
For instance the Datadog repository contains some unsigned packages.
With the new default value for `gpgkey` to true, the package installation fails.
I think the previous behavior makes more sense:
Unless explicitly set, gpgcheck should be true only if gpgkey is set.