Interface mtu property

I use jumbo frames on a few subnets so I've taken a stab at adding a mtu property to the network_config type and corresponding support in the redhat provider.  This has only been tested on SL6.4.  I haven't yet tried to implement debian family provider support but am happy to take that on before this PR is merged if this type API change is potentially mergable.

    # puppet resource --modulepath=`pwd` network_config eth0
    network_config { 'eth0':
      ensure    => 'present',
      family    => 'inet',
      ipaddress => '10.10.10.10',
      method    => 'static',
      mtu       => '9000',
      netmask   => '255.255.255.0',
      onboot    => 'true',
      options   => {'HWADDR' => '00:1F:C6:16:0A:91', 'IPV6INIT' => 'no', 'NM_CONTROLLED' => 'no', 'TYPE' => 'Ethernet'},
    }