Make the WebApps logger configurable

[victorhurdugaci]

Right now, in it's rough implementation, it only accepts a parameter. It should be configurable through the configuration system (options?) and you should be able to set an environment variable to configure the max file size, for example.

cc @pakrym

[muratg]

@victorhurdugaci Let's have a look when you're back.

[victorhurdugaci]

I think pakrym fixed this...

[muratg]

@pakrym remember fixing this?

[muratg]

@pakrym If this isn't already done, we can move this out.

[pakrym]

Done as part of #477
See https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Extensions.Logging.AzureAppServices/AzureAppServicesDiagnosticsSettings.cs