Increase log level on authenticate commands done for internal authentication

When using authentication with replica sets or sharding, there is a lot of messages in the logs on the mongods that look like

Wed May  9 14:48:11 [conn241]  authenticate: { authenticate: 1, nonce:

"b489d14f5732414", user: "__system", key:

"7a6ad96f3a481312fe03e9ed5bf59ae5" }

These messages are from the internal authentication used for nodes and mongos to authenticate their connections to each other. We probably still want to log authentication requests from the user at the default log level, but the internal authentication messages just fill the logs with a lot of unnecessary noise.