Added ssh_private_ip config to enable use of VM private IP addresses

[Ancillas]

When using an Azure VNet, public endpoints cannot be used to reach virtual machines.

This commit adds an ssh_private_ip config option which accepts a boolean value. When true, the private IP address of Azure VMs will be used when executing remote provisioning commands, instead of the public endpoint IP.

[msftclas]

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[msftclas]

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[paulmey]

ssh_private_ip -> use_private_ip, also, let's do SSHPrivateIp -> UsePrivateIP to keep it consistent. (This should also made to work similarly for Windows VM's, once I get those over to using WinRM instead of CustomScript.)

[Ancillas]

I rebased my branch before I pushed, but I recall originally using use_private_ip. For some reason, I had a problem in that my config was never picked up, and was always empty. That's why I ended up changing to SSHPrivateIp.

It's highly likely I was doing something wrong. I'll try changing the value around and re-testing at my next opportunity.

[paulmey]

The above comment should fix the build. Also please run go fmt to get rid of the spaces/tabs mix.
Thanks for contributing!

[Ancillas]

Thanks for the feedback. I'll get the files formatted correctly as soon as I can.