Eliminate circular import dependency

`ansible.callbacks` and `ansible.utils.display_functions` had circular dependencies.

I removed the latter dependency, because it was depending on something at a higher level which is a smell. I did this by adding a `set_display_callback` function, so that display callback can be set at runtime rather than import-time. This is also more flexible.

I also changed the imports of ansible stuff in `ansible.callbacks` so that instead of using implicit relative imports (which are hard to understand) to explicit absolute imports. Explicit is better than implicit.