Fix index creation in MongoDB

Currently indexes on (users and groups) directories are created by taking into account the schema name whereas the storage (and the queries) use the schema prefix.

As a result, we have in MongoDB :

• the data like

  {
      "_id": {
          "$oid": "61f9034b7f26b85e23903b6d"
      },
      "description": "Group of users with read access rights",
      "grouplabel": "Members group",
      "groupname": "members"
  }
  

• the index with the name group:groupname_1

So the index "group:groupname_1" is never used and the queries on this field could be extremely slow

Expected behavior: with the default "group" schema definition, the index name should be groupname_1

The same issue is present at the repository level. Both cases will receive the fix.