Allow multiple nics

#### Description

For my network project, I need to generate a packer template for a firewall. To pre-configure it correctly, I need two NICs inside the packer template. Currently it is only possible to use one.

I am in the process of extending the hyperv plugin to support multiple nics. Feedback is welcome. Pull request will come.

#### Use Case(s)

Provisioning hyperv images for servers that require multiple nics, e.g. firewalls, routers, etc.

#### Potential configuration

I want to add the following block to the configuration:
````
network_adapter {
name = "Adapter 1" // optional, name of the network card
mac_address = "00dd12ed11dd" // optional, mac address of the network card
enable_mac_spoofing = true // optional, allow the guest os to spoof the mac address for the network card
vlan_id = "1" // optional, vlan id for the network card
switch_name = "wan" // optional, name of the switch
switch_vlan_id = "1" // optional, VLan ID for the switch if generated
}
````

The old (embedded) settings will still be available to keep compatibility with old configurations. If old and new options are used at the same time a warning will be issued. In this case, old (embedded) options will be inherited to new defined adapters, e.g.

````
mac_address = "001122334455"
enable_mac_spoofing = true

network_adapter {
name = "wan"
}

network_adapter {
name = "lan"
mac_address = "112233445566"
}
````

In this case both network adapters will inherit the enable_mac_spoofing value. The mac_address will be inherited to the adapter named "wan" but not to the one named "lan", since it defines mac_address itself.

Instead of inheriting the embedded settings the following behaviours would also be possible (please give feedback):

- stop with an error "Use either the new network_adapter block or the embedded settings"
- issue a warning and ignore the embedded settings
- make the first network adapter from the embedded settings, make additional ones using the network_adapter block.

#### Potential References

Other plugins also feature the addition of multiple network adapters, e.g. proxmox or amazon.