Add migration path from 1 to 2 for missing gid

NIS placeholders didn't have a GID in json in v0.19*. To support
upgrading without issues we add the GID now to comply with our schema.

Our schema was added in v0.20 were the GID issue was also fixed but
there was no migration added for old manifest files.
Merge pull request #297 from SUSE/add_migration_for_missing_gid

Add migration path from 1 to 2 for missing gid