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.
