Allow setting mac address manually

[SamK]

Hi there,

Each time I recreate a VM, the MAC address changes and the DHCP server increments the hostname with a number.
This result is that after a few tries, my server which should be resolved with server01 is instead resolved with server01-8.

dmacvicar/libvirt allows the MAC address to be set with the the network_interface.mac field.

Can I do the same with terraform-libvirt-vm?