Add missing className option to generated associations.

Child/Parent associations need to specify the className key or the
incorrect classname will be inferred at runtime causing errors.

We're using the alias as a proxy for classname, as we can't use the
actual classname as bake may be using a base class replacement.

Refs #3791
Correctly generate classnames for plugins as well.

Refs #3791
Merge pull request #3812 from cakephp/issue-3791

Add missing className option to generated associations.