Create mongodb connection on servlet startup

Create the mongo connection on servlet startup so that
we can fail on startup if something is not correctly
configured.

Moved MongoClient and DB instances into a MongoConnection
object that is injectable.
Use an embedded mongo server when running integration tests.
Allow embedded mongodb test properties to be overridden

Mongo test properties can be overridden by adding a embedded-mongo.properties
file to the classpath. This is primarily useful if you need to override the
default port of 12345.
Fixing checkstyle
Merge pull request #658 from candlepin/mstead/mongo-refactor

Create mongodb connection on servlet startup