Add support for flat config file which specifies flags to run binary with.

• YAML structure.
• CLI and CONFIG file should have a very obvious relationship to each other.
  That is to say, should be able to pull from config file and turn this into a line to run k3s with these args

To expand on the CLI and CONFIG file relationship. The concern is that if the config file is different enough from the cli flags, it makes things difficult to understand. Docs would be required for both the config and cli with different terms and this just makes things confusing.

Ideally, can run K3s strictly from a config file and to change things dynamically supply command line flags to change things [if we do this how does config file get updated (and should it)].