NoCloud's required configuration types is unnecessarily inconsistent across different sources

# Bug report

In NoCloud, meta data and user data are sometimes optional, and sometimes not - it depends on the source.

This design choice was both 1) unnecessary and 2) inconsistent. We should standardize which configurations (out of network-config, meta-data, vendor-data, user-data) NoCloud requires across all of its sources: that is, make it optional in all cases.

See [this comment](https://github.com/canonical/cloud-init/pull/5521#discussion_r1683614347) for more context.