Optionally allow java to not be managed via the java cookbook.

There are use cases where Java is already installed or needs to be managed in a custom way. There should be a way for someone using the maven cookbook to tell it to not manage java for them. The Jenkins cookbook has taken a similar approach.

I will also be documenting guidelines for cookbook maintainers who depend on the java cookbook.

In reference to https://github.com/agileorbit-cookbooks/java/issues/190