MongoDB replica sets

Brooklyn entities to start and manage MongoDB replica sets.

Test with `mvn clean install -PIntegration -Dtest=MongoDbReplicaSetIntegrationTest` from `software/nosql`.

A problem with `mongod` (that appears to be OS X specific) means that sometimes servers in replica sets will fail to start, and thus the integration tests will fail when asserting sets have a particular size. I'm unaware of any workaround for this other than to run the tests on other operating systems or to change the assertions.