Use $labs_finger as default $master finger in salt role

This fixes an error in labs where the pick() function failed
puppet runs:

Sep 15 15:05:39 wikimetrics-staging1 puppet-agent[25708]: Could not retrieve catalog from remote server: Error 400 on SERVER: Must provide non empty value. on node wikimetrics-staging1.eqiad.wmflabs

Change-Id: Id425238a0bd48bffec054c29b30b9019a785789b