Cleanup live hack from wmf-deployment r53208 a bit: DB selection using load balancer.

Cleanup live hack from wmf-deployment r53208 a bit: DB selection using load balancer.

Should fix a couple regressions or possible regressions:
* Hack was pulling the main LB's DB_SLAVE connection although we were deliberately opening a new connection. Could have caused problems when we were fetching other things from the local database in the middle of an export.
* We no longer lose the connection timeout override.

However we still have a regression:
* --server option is ignored (but we're now pulling from the 'backup' group, so we at least have some kind of consistency :)