DataSourceNoCloud: allow setting user-data and meta-data in config
This allows a single file to declare and activate this data source.
This could come from:
* cloud-config-url on kernel cmdline
* /etc/cloud/cloud.cfg.d
* debian preseed of 'cloud-init/local-cloud-config'
Also here is
* some tests
* a small fix to parse_cmdline_data found when writing those tests.