Make parameter list generation consistent using urlencode().

Ensure that each client is generating the parameter list
correctly by using the urllib.urlencode method.

Fixes: bug 1086574
Change-Id: I9008a57fe94ccec1f3bea95f314860ff8017cb42
diff --git a/tempest/services/compute/xml/volumes_extensions_client.py b/tempest/services/compute/xml/volumes_extensions_client.py
index 0fbc070..871f2e5 100644
--- a/tempest/services/compute/xml/volumes_extensions_client.py
+++ b/tempest/services/compute/xml/volumes_extensions_client.py
@@ -17,6 +17,7 @@
 
 import time
 from lxml import etree
+import urllib
 
 from tempest import exceptions
 from tempest.common.rest_client import RestClientXML