Move license default behavior into library

Moves the handling of `['newrelic']['license']`, `['newrelic']['server_monitoring']['license']`, and `['newrelic']['application_monitoring']['license']` into a library. This defers the calculation of which value to use until the actual runtime, allowing wrapper cookbooks to set `['newrelic']['license']` and still have the value they set be usable for the other two.

This solves the complaint in https://github.com/escapestudios-cookbooks/newrelic/issues/101.

For what it's worth, you can see the successful result in the TK suite, but your tests mostly don't exist or don't seem to work, and there aren't any serverspec tests, so I didn't bother adding this to one of them.