make mountpoint a namevar for ext4mount

This means that you can specify

    ext4mount {'/mnt/uploads':
        mountoptions => 'defaults',
        disk         => '/dev/sda1',
    }

instead of previously needing to be explicit with the mountpoint:

    ext4mount {'/mnt/uploads':
        mountpoint   => '/mnt/uploads',
        mountoptions => 'defaults',
        disk         => '/dev/sda1',
    }

You can still specify the mountpoint separately if you wish.