Support authenticationDatabase.
Solves similar issue as in https://jira.spring.io/browse/DATAMONGO-789
Merge pull request #115 from greyfairer/axon-2.1.x

AXON-231 MongoDB: support login via different (e.g. admin) authentication database.

The MongoTemplate implementations now support authentication against another database that will store the actual data.

Issue #AXON-231 Fixed