SuSE based distributions get the `gid` from `users` not the `gid` from it's name. Fixes #7003.

Full explanation about why is this can be seen here:

  https://github.com/saltstack/salt/issues/7003#issuecomment-23813071
Use the `requires_system_grains` decorator from salt-testing
Merge pull request #7048 from s0undt3ch/issues/7003

SuSE based distributions get the `gid` from `users` not the `gid` from it's name