MongoDb: Change init properties

[remi128]

mongo allows to init the datastore either by connection_string or by driver properties. If connection_string is set, driver properties are omitted.
Switch init to driver properties, since more possible options.

See http://vertx.io/docs/vertx-mongo-client/java/#_configuring_the_client

check wether that way the pool exception can be avoided during massinserts