Only allow empty strings for gid/uids
Allow empty string for gids in the group scope

NIS adds the placeholder "+:::" to /etc/group which didn't work
with the previous schema.
Add integration test checks for NIS placeholders

NIS adds a placeholder to the /etc/passwd and /etc/groups which have
empty values except a "+" for the name.
Merge pull request #167 from SUSE/adapt_group_schema_for_nis

Adapt group schema for nis