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.