[JobQueue] Use target wiki configuration for some key functions.

* getQueueTypes() now gets the config of the target wiki.
  Previously, for WMF, if an extension using jobs was not
  on aawiki, those job queues would not be seen by nextJobDB.
* Also fixed nextJobDB.php so that it no longer only considers
  jobs types known to aawiki for picking a DB for default jobs.
* Note that $wgJobTypesExcludedFromDefaultQueue should be global.
* This adds a SiteConfiguration::getConfig() function, which calls
  a new getConfiguration.php script.

Change-Id: I7e6904ead17efa407291f423a2b18e3c866d55fd