add(coreos-base/oem-niftycloud): added initial oem for niftycloud

These are the initial oem scripts for niftycloud.
NIFTY Cloud is one of the most popular public cloud service in Japan.

cc: @kelseyhightower 
Sorry, my unit is different from https://github.com/coreos/init/blob/master/systemd/network/yy-vmware.network.

Following two units resolved my issue...

```
    - name: dhcp.network
      content: |
        [Match]
        Name=ens*

        [Network]
        DHCP=yes

        [DHCP]
        RequestBroadcast=true
```

```
    - name: dhcp.network
      content: |
        [Match]
        Name=ens*

        [Network]
        DHCP=yes

        [DHCP]
        UseMTU=true
```