support reading network interface config from DataSourceNoCloud

document usage of DataSourceNoCloud from vfat or iso disk.

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diff --git a/doc/nocloud/README b/doc/nocloud/README
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+The data source 'NoCloud' and 'NoCloudNet' allow the user to provide user-data
+and meta-data to the instance without running a network service (or even without
+having a network at all)
+
+You can provide meta-data and user-data to a local vm boot via files on a vfat
+or iso9660 filesystem.  These user-data and meta-data files are expected to be
+in the format described in doc/example/seed/README .  Basically, user-data is
+simply user-data and meta-data is a yaml formated file representing what you'd
+find in the EC2 metadata service.
+
+Given a disk 12.04 cloud image in 'disk.img', you can create a sufficient disk
+by following the example below.
+
+## create user-data and meta-data files that will be used
+## to modify image on first boot
+$ { echo instance-id: iid-local01; echo local-hostname: cloudimg; } > meta-data
+
+$ printf "#cloud-config\npassword: passw0rd\nchpasswd: { expire: False }\nssh_pwauth: True\n" > user-data
+
+## create a disk to attach with some user-data and meta-data
+$ genisoimage  -output seed.iso -volid cidata -joliet -rock user-data meta-data
+
+## alternatively, create a vfat filesystem with same files
+## $ truncate --size 2M seed.img
+## $ mkfs.vfat -n cidata seed.img
+## $ mcopy -oi seed.img user-data meta-data ::
+
+## create a new qcow image to boot, backed by your original image
+$ qemu-img create -f qcow2 -b disk.img boot-disk.img
+
+## boot the image and login as 'ubuntu' with password 'passw0rd'
+## note, passw0rd was set as password through the user-data above,
+## there is no password set on these images.
+$ kvm -m 256 \
+   -net nic -net user,hostfwd=tcp::2222-:22 \
+   -drive file=boot-disk.img,if=virtio \
+   -drive file=seed.iso,if=virtio
+
+Note, that the instance-id provided ('iid-local01' above) is what is used to
+determine if this is "first boot".  So if you are making updates to user-data
+you will also have to change that, or start the disk fresh.
+
+
+Also, you can inject an /etc/network/interfaces file by providing the content
+for that file in the 'network-interfaces' field of metadata.  Example metadata:
+  instance-id: iid-abcdefg
+  network-interfaces: |
+    iface eth0 inet static
+    address 192.168.1.10
+    network 192.168.1.0
+    netmask 255.255.255.0
+    broadcast 192.168.1.255
+    gateway 192.168.1.254
+  hostname: myhost
+