This Spark trait is used for mapping a given userName to a set of groups which it belongs to.
This is useful for specifying a common group of admins/developers to provide them admin, modify
and/or view access rights. Based on whether access control checks are enabled using
spark.acls.enable, every time a user tries to access or modify the application, the
SecurityManager gets the corresponding groups a user belongs to from the instance of the groups
mapping provider specified by the entry spark.user.groups.mapping.