VMTasks.dd_load: No random input for write test
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

openstack/rally,openstack/rally,openstack/rally,openstack/rally