Two new configuration options have been exposed to allow users to specify
an existing virtual network: virtual_network_name and
virtual_network_resource_group_name.

 * virtual_network_name: name of the virtual network to attach a Packer VM
 to.

 * virtual_network_resource_group_name: name of the resource group that
 contains the virtual network.  This value is optional.  If the value is
 not specified, the builder queries Azure for the appropriate value.  If
 the builder cannot disambiguate the value, a value must be provided for
 this setting.

 * virtual_network_subnet_name: name of the subnet attached to the virtual
 network.  This value is optional.  If the value is not specified, the
 builder queries Azure for the appropriate value.  If the builder cannot
 disambiguate the value, a value must be provided for this setting.