fix setting log level

Allow the console log level to be customisable in the app or config files

The old config key log_level was I think abandoned when switching to lager. Now I've changed it to affect the level of lager_console_backend. I was not sure if it should also set the "console.log". Probably not as it is useful to have at least one file to contain all messages.

Anyway I guess the below page needs to be updated and cleared from any reference to basho_bench_log.erl
http://docs.basho.com/riak/latest/cookbooks/Benchmarking/