Update to Mongo 3.0

edX uses Mongo 3.0 in production. Devstack should reflect this fact. Use the [existing configuration](https://github.com/edx/configuration/tree/master/playbooks/roles/mongo_3_0) to pull, and configure, the container correctly. Specifically, note that we use the [WiredTiger storage engine](https://docs.mongodb.com/manual/core/wiredtiger/).