Attr. modules: unixGroupName can work without GID

Addition to commit 678b82f74ff0b7124da622243fea48bd226ef762 (Now you can
set unixGroupName without unixGID if unixGIDNamespace isn't set on
facility.) which works only for resource's attributes. This one work for
group's attributes too.
Merge pull request #63 from licehammer/unixGroupsWithoutGID

Attr. modules: unixGroupName can work without GID