add support for optional ethernet mac address in hardware config

[ComrexSteve]

We had a need to generate OVA/OVF with the Ethernet MAC address as a parameter. This adds support for that with an optional "address" specified in the ethernet hardware config yaml.

    ethernet1:
        type: ethernet
        subtype: VmxNet3
        network: vm_network1
        address: !param mac_primary

[oliverkurth]

This looks good to me, thank you! Please fix the issue with the DCO, and I will merge this.

Optionally, you can create a test for this too.

[oliverkurth]

I almost forgot this - thank you! Merging.