Previously, if a user defined a MongoDbFactory bean but did not define
a client bean, MongoDataAutoConfiguration would back off leaving the
context without a MongoTemplate, etc.

This commit reworks the auto-configuration so that only the
auto-configuration of a MongoDbFactory is dependent on the existence
of a Mongo client bean. Auto-configuration of the other components
that depend on a MongoDbFactory will now continue in the absence of a
Mongo client bean.

Closes gh-17416