VMTasks.dd_load: No random input for write test

For a dd write test it does not matter whether input data comes from
/dev/random or from /dev/zero.
To speed up the creation of test data this patch replaces /dev/random as
write test source with /dev/zero.

Co-Authored-By: Daniel Haupt <daniel.haupt@sap.com>

Change-Id: I3bcb25ec82920fa340e1bbe08e2d6f473de5c5bd

rally/plugins/openstack/scenarios/vm/vmtasks.py

@@ -438,7 +438,7 @@ EOF
 additive_dd() {
     local c=${1:-50} # Megabytes
     local file=/tmp/dd_test.img
-    local write=$(get_seconds "dd if=/dev/urandom of=${file} bs=1M count=${c}")
+    local write=$(get_seconds "dd if=/dev/zero of=${file} bs=1M count=${c}")
     local read=$(get_seconds "dd if=${file} of=/dev/null bs=1M count=${c}")
     local gzip=$(get_seconds "gzip ${file}")
     rm ${file}.gz