Fixes for stage4 users/groups options

cc: @monsieurp

LGTM, please proceed and merge those patches

added 4 commits

• ae7d28b8 - catalyst/stage4: fix handling of groups, users and keys
• 214bb3d8 - catalyst/stage4: don't run split on a list
• 430a699c - catalyst/stage4: don't split a single entry
• a21d590f - catalyst/stage4: improve log message

Compare with previous version

merged