Improve MongoDB TestDriver

* pom.xml: Use 1.45 release of flapdoodle embed mongo
* pom.xml: Bump mongo java driver to 2.12.1
* GoraMongodbTestDriver: Use MongoDB 2.6 release
* GoraMongodbTestDriver: Start with a random free port (and inject this random port into Hadoop configuration)
* gora.properties: Cleanup file to only keep mongo properties