Public IP support for existing vnet deployment

[vlesierse]

While using this driver I ran into the scenario that I would like to provision my VM using Kitchen in an existing VNET on Azure. However the VM in the VNET is accessible from my local network because there is no gateway in between. There for I would like the possibility to expose a public IP when the VNET is specified.

Implementation details

• Config parameter public_ip
• Using ERB for rendering the ARM template to have better support for conditions inside the template.
• Moved the ARM templates to separate files, however it is also possible to include the template in the same azurerm.rb file

[stuartpreston]

Thanks very much for this contribution. I'm just testing it out and will merge shortly.