user module raises warning about real_gid

Originally created by @groente on #16213 (Redmine)

stone complains with:

Warning: Unknown variable: ‘real_gid’. at /etc/puppet/code/modules/user/manifests/managed.pp:139:8

Feature Branch: puppet-user:bugfix/16213-warning-about-real_gid

Parent Task: tails/sysadmin#16165

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