Add retries for deleting VMs

Change-Id: I5f4d2b5577b3ff252ec69603c4d77656d21022c5

.gitreview

@@ -1,4 +1,4 @@
 [gerrit]
 host=review.openstack.org
 port=29418
-project=openstack/vmtp.git
+project=x/vmtp.git

vmtp/compute.py

@@ -230,7 +230,12 @@ class Compute(object):
         return False
 
     def delete_server(self, server):
-        self.novaclient.servers.delete(server)
+        for _ in range(1, 5):
+            try:
+                self.novaclient.servers.delete(server)
+            except novaclient.exceptions.NotFound:
+                break
+            time.sleep(2)
 
     def find_flavor(self, flavor_type):
         try: