Provide unified KEDA core configuration

A clear and concise description of what you want to happen.

Use-Case

When you run KEDA in a 12-factor environment, it's sometimes easier to configure it with environment variables (not commandline flags), in other cases, a pre-canned config file is more useful, and of course, we already have commandline flags, so those are certainly more useful to some people

I think we should support all 3 ways of configuring the KEDA controller (not necessarily the metrics API, but that could be considered)

Specification

• Change the stdlib flags package usage to use viper, or a similar library that can draw configuration values from multiple sources