Add network proxy configuration settings

[jongiddy]

Allow configuration of a network proxy for external calls to network. Adds the ML cluster hostnames to no_proxy to ensure connections work between VirtualBox nodes.

Fixes #82

[grtjn]

Looks great, thanks. Just wanna check 2 things myself: what happens when you haven't installed proxyconf yet, and what if you did, but omitted those new properties. It will probably work just fine, but just to make sure..

[jongiddy]

After suggesting in #82 that my fix just needed a guard expression, I forgot to add it. It now works for 3 scenarios:

1. No proxy set, it ignores the plugin status and extra config.
2. Proxy set, but plugin not installed, it emits an error message to install the plugin.
3. Proxy set and plugin installed, it configures the proxy on the hosts.

[grtjn]

Thanks, looks great!

[grtjn]

Thanks, works nice the warning!