Add DatasourceOVF network-config propery to Ubuntu OVF example

[megian]

Cloud-init includes the capability to take the network-config from a
separate key. This removes the need to merge the network config in
the user-data and make it more transparent in some cases.

Reference:

cloud-init/cloudinit/sources/DataSourceOVF.py

Line 557 in 42b938e

network_props = ["network-config"]

Test Steps

• Create a template using the OVF example

• Instantiate a VM from that template

• Set the property key network-config to

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• Check that if the content has been written to /etc/netplan/50-cloud-init.yaml inside the VM operating system

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly (N/A because it is just an example file)
• I have updated or added any documentation accordingly (N/A because it is just an example file)

[TheRealFalcon]

Thanks. I left one inline suggestion.

[TheRealFalcon]

Thanks!