Introduce Sane Log Levels

The lager level philosophy of riak_test should be as follows:

* `info` - anything that would be helpful in understanding what happened during a test
* `debug` - anything that would help a developer in writing or debugging a test

This issue will be resolved when:
* Those levels make it into the documentation somewhere obvious
* All existing log messages are converted to lager:info
* Make sure that the `rt_lager_level` setting actually does something, or make it move obvious what it does by renaming to something better.