Add Dev Support to Network settings

[ALameLlama]

Currently trying to set a network interface when you don't have an eth0 will cause it to fail.

[root@homestead testing-homestead]# vagrant up
Bringing machine 'testing-homestead' up with 'libvirt' provider...
There are errors in the configuration of this machine. Please fix
the following errors and try again:

Libvirt Provider:
* network configuration 2 for machine testing-homestead is a public_network referencing host device 'eth0' which does not exist, consider adding ':dev => ....' referencing one of enp2s0

This allow passing a dev e.g:

networks:
    - type: "public_network"
      ip: "192.168.1.72"
      bridge: "enp2s0"
      dev: "enp2s0"

[ALameLlama]

If we've happy with this, I can also update the docs

[svpernova09]

Thanks