The work done in core to move globals behind a config object is the first step in being able to access the configuration of another wiki. We need to take this to the next step and actually read in the cirrussearch configuration of another wiki.
Ideally this should work both in wmf prod and in mediawiki-vagrant. This is *NOT* currently supported and we will have to figure out how to make it work. Currently the result of running InitializeSettings.php is cached but these are just the $wmgCirrusSearch* variables and not the $wgCirrusSearch* variables that we want.