scripts: Add Ubuntu cloud-init data

Add cloud-init data for Ubuntu and introduce a convenience script that
can be used to generate cloud-init disk images for manual testing.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

test_data/cloud-init/ubuntu/network-config

@@ -0,0 +1,12 @@
+network:
+  version: 1
+  config:
+    - type: physical
+      name: eth0
+      mac_address: 12:34:56:78:90:ab
+      subnets:
+         - type: static
+           address: 192.168.2.2/24
+           gateway: 192.168.2.1
+           dns_nameservers:
+              - 192.168.2.1