(maint) Update mysql_user provider from #1004 for #1036

Author: hunner

lib/puppet/provider/mysql_user/mysql.rb

@@ -171,7 +171,7 @@ def plugin=(string)
       sql << " WHERE CONCAT(user, '@', host) = '#{@resource[:name]}'"
     end
 
-    mysql([defaults_file, system_database, '-e', sql].compact)
+    self.class.mysql_caller(sql, 'system')
     (plugin == string) ? (return true) : (return false)
   end