Make log_level entry in rebar.app work again

[richcarl]

The log_level app setting had become ignored during refactorings of old. This makes it take effect again and syncs its standard setting with that of with rebar_log:default_level(). We add rebar_log:atom_to_level/1 to avoid exposing the level numbers used internally.

[tsloughter]

I think that was meant to be removed from rebar.app since you'd change it in config or environment and not by rebuilding rebar3.

Not that this isn't an ok change since its not like it hurts to have I guess.

[richcarl]

I think that was meant to be removed from rebar.app since you'd change it in config or environment and not by rebuilding rebar3.

Yeah, I thought about dropping it, but it's a nice and easy way to rebuild rebar with your default level of choice. Also, the only other way to change the level currently is by setting env variables like DEBUG or QUIET.