Improve config file mechanism

[r10r]

I really like the crio config command to generate the configuration file based on the passed flags and environment variables.
This makes it very convenient to generate a matching config file.
lxcri should support a similar mechanism for convenience.

• Replace the environment file /etc/default/lxcri with a YAML config file in /etc/lxcri/lxcri.yaml.
• Add a cli cmd lxcri {args} config to create and edit the config file.
• Also add a flag and environment variable to overwrite the default location of the config file path, to support multiple environments.