Fix mapping of groups for frontend.

Created by: bbockelm

With this, we can change the frontend matching expression to:

__import__("CMSGroupMapper").is_local_group(job["CMSGroups"], glidein["attrs"]["GLIDEIN_CMSSite"])

for the local user group. Then, we will just need a small CRAB3 change to do VOMS-based local group mappings.