Optimize the dd command to improve performance

Use if=/dev/zero: Instead of reading from /dev/urandom,
which is slow for generating random data, use /dev/zero
to write zeros to the disk. Zeros are generated much faster.
Omit conv=fsync: The conv=fsync option forces dd to
synchronize the data after each write, which can be slow for
large writes.

Change-Id: Id845c60c39072a03171fa5906461eb254c2736a7
diff --git a/zuul.d/manila-tempest-jobs.yaml b/zuul.d/manila-tempest-jobs.yaml
index f2829bc..6214867 100644
--- a/zuul.d/manila-tempest-jobs.yaml
+++ b/zuul.d/manila-tempest-jobs.yaml
@@ -157,6 +157,7 @@
               backend_names: LONDON,PARIS
               multi_backend: true
               image_password: manila
+              dd_input_file: /dev/urandom
 
 - job:
     name: manila-tempest-plugin-zfsonlinux