Support fileserver_list_cache_time in master configuration

salt/files/master.d/f_defaults.conf

 # has a very large number of files and performance is impacted. Default is False.
 {{ get_config('fileserver_limit_traversal', 'False') }}
 
+# Salt caches the list of files/symlinks/directories for each fileserver backend
+# and environment as they are requested, to guard against a performance bottleneck
+# at scale when many minions all ask the fileserver which files are available
+# simultaneously. This configuration parameter allows for the max age of that
+# cache to be altered.
+#
+# Set this value to 0 to disable use of this cache altogether, but keep in mind
+# that this may increase the CPU load on the master when running a highstate on
+# a large number of minions.
+#
+# Rather than altering this configuration parameter, it may be advisable to use
+# the fileserver.clear_list_cache runner to clear these caches.
+{{ get_config('fileserver_list_cache_time', '20') }}
+
 # The fileserver can fire events off every time the fileserver is updated,
 # these are disabled by default, but can be easily turned on by setting this
 # flag to True