Bug 1010723 - Only run lscgroup once for check_users

https://bugzilla.redhat.com/show_bug.cgi?id=1010723

Running lscgroup once per user gets expensive, so run it once and
cache the results.  This is also more in line with how quota info
and other data is gathered in oo-accept-node.
Merge pull request #3678 from a13m/bugzilla/1010723

Merged by openshift-bot