[docs]: NoCloud missing mandatory file information

[stmccartneyncc]

Documentation request

The NoCloud doc page is missing the fact that both user-data and meta-data are required in order for NoCloud to work successfully, and that the other two files are optional. If you don't have both user-data and meta-data you get the error "device /dev/sd* with label=cidata not a valid seed"

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Reported from: https://cloudinit.readthedocs.io/en/latest/reference/datasources/nocloud.html