161 os network config output mode

[benformosa]

Fixes #161

This change:

• updates the show_os_network_config function to support the output-mode option.
• adds a new subcommand show-os to display only the OS network information.

I have a few things that I was unsure about:

• Naming - I've created a new wrapper function do_show_os_network_config. I've only used the do_ prefix because I've seen it elsewhere. It's not ideal, and the two functions could possibly be merged.
• Updating the oci-network-config command. In my testing, I couldn't seem my new subcommand.

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