Update MongoAutoConfiguration to be conditional on a missing
MongoDbFactory bean. The assumption is that if the user has declared
a MongoDbFactory they will either use it directly, or they will also
register a Mongo bean.

If the MongoDbFactory class cannot be found the existing Mongo
auto-configuration still applies. This ensures that users that do
not have Spring Data can still access a Mongo bean.

Fixes gh-1341