Add indexes for user and group directories

The following indexes are being added to Nuxeo systems because we see queries generated by the platform using these non-indexed fields in the DD APM monitoring.  We'd like to add these to the platform to better support deployments having large user/group directories.

1. username in userDirectory collection
2. userId in user2group collection
3. groupId in user2group collection
4. groupname in groupDirectory collection
5. parentGroupId in group2group collection
6. childGroupId in group2group collection