Fix wfWaitForSlaves() for foreign DBs

We thought we'd fixed it previously, but the LB now connects to the
foreign slaves and tries to select $wgDBname. Fixing this by having
doWait() open connections that don't select any DB name, by passing
the empty string as the DB name. This is an undocumented feature
in the DatabaseMysql constructor, apparently.

Change-Id: Icc5594143a3eaa31170a7fc8c93d429c1a7dd575