Allow db tags to be used

The Mongo driver will then be able to choose the best
host based on tag values, e.g. that in the same
data centre.
Merge branch 'mongo-tags' into stage

Conflicts:
	src/main/java/org/atlasapi/MongoSecondaryReadPreferenceBuilder.java
	src/test/java/org/atlasapi/MongoSecondaryReadPreferenceBuilderTest.java