Handle nova image preallocation mode

Image preallocation allows storage to be fully allocated at instance
start. This should significantly improve performance on writes to new
blocks and may even improve I/O performance to prewritten blocks due
to reduced fragmentation.

Change-Id: Ic285d6bc07dcee65db2dffca9d2730705761121c
diff --git a/tests/pillar/compute_cluster.sls b/tests/pillar/compute_cluster.sls
index 28ded0c..4bbcfba 100644
--- a/tests/pillar/compute_cluster.sls
+++ b/tests/pillar/compute_cluster.sls
@@ -13,6 +13,7 @@
     report_interval: 60
     vnc_keymap: en-gb
     resume_guests_state_on_host_boot: True
+    preallocate_images: space
     bind:
       vnc_address: 127.0.0.1
       vnc_port: 6080