allow setting macaddress

This is needed to add a physical interface to the bridge with a MAC.
Example:

```
  network::bridge::static { 'br1':
    ensure    => 'up',
    ipaddress => $::ipaddress,
    netmask   => $::netmask,
  }
  network::if::bridge { 'em1':
    macaddress   => $macaddress_em1,
    ensure       => 'up',
    ethtool_opts => '',
    bridge       => 'br1',
  }```