Add nr parameter to file_service_type

Right now we switch for every IO, add a postfix that allows to switch
for every 'x' number of ios.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 1374d05..f9e3fc7 100644
--- a/HOWTO
+++ b/HOWTO
@@ -265,6 +265,11 @@
 			roundrobin  Round robin over open files. This
 				is the default.
 
+		The string can have a number appended, indicating how
+		often to switch to a new file. So if option random:4 is
+		given, fio will switch to a new random file after 4 ios
+		have been issued.
+
 ioengine=str	Defines how the job issues io to the file. The following
 		types are defined: