User/Groups already exists exception mapper
Adding create group throws exception
Send BAD_REQUEST instead of CONFLICT on user/group already exists exception
BAD_REQUEST response returns a message key that can be localized.
Merge pull request #253 from emorency/feature/OPAL-1878-user-groups

OPAL-1957: Support UserAlreadyExists and GroupAlreadyExists exceptions
Calling new constructor of MongoDbDatasource