QA: also run gnt-cluster verify-disks

The bug recently reported by Apollon Oikonomopoulos was missed because
we don't test this command at all.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

qa/qa_cluster.py

@@ -153,6 +153,7 @@ def TestClusterOob():
 def TestClusterVerify():
   """gnt-cluster verify"""
   AssertCommand(["gnt-cluster", "verify"])
+  AssertCommand(["gnt-cluster", "verify-disks"])
 
 
 def TestJobqueue():