Support custom and private networks for Azure

[r4victor]

Closes #1672

This PR adds support for custom Azure networks specified via the vpc_ids mapping and instances without public IPs specified via public_ips: false:

type: azure
  ...
  regions: [westeurope]
  public_ips: false
  vpc_ids:
    westeurope: test-networks-rg/test-network
  creds:
    type: default

public_ips: false requires a NAT Gateway or a VNet peering.