Add authentication using authSource

[Cosmiiko]

The Mongo connection string standard states that, following the
mongodb://[username:password@]host1[:port1][,...hostN[:portN]][/[defaultauthdb][?options]] format,

(...) the client will attempt to authenticate the user to the authSource. If authSource is unspecified, the client will attempt to authenticate the user to the defaultauthdb. And if the defaultauthdb is unspecified, to the admin database.

This PR implements auth from the authSource option, with greater priority than defaultauthdb, as stated above.