config-drive-v2: populate metadata['public-keys'] from 'public_keys'
other datasources populate 'public-keys' rather than 'public_keys' and there is a more complete handler in the base DataSource. So, to take advantage of that, have DataSourceConfigDrive copy public_keys to public-keys, and remove the 'get_public_ssh_keys' from the DataSourcEConfigDrive.