Updated anonymous NFS guidance

Update RHEL6 XCCDF to reflect -1 value may break interop;
Update 6 & 7 to reflect UID/GID max from login.defs
Remove RHEL7 CCE

Traceback to downstream Red Hat Support ticket # 01192254.
Merge pull request #236 from shawndwells/anonnfs-take2

Updated anonymous NFS guidance