Move xml common code into the common dir

This commit moves tempest.services.compute.xml.common to the proper
location in tempest.common and renames the file to xml_utils.py. This
module was used in every client not just compute so the appropriate
place for this is in the common dir.

Change-Id: I42a8338fd507c1224fa9fd2cdc5c1b0700f7a26a
diff --git a/tempest/services/identity/v3/xml/policy_client.py b/tempest/services/identity/v3/xml/policy_client.py
index bf4cce7..e903089 100644
--- a/tempest/services/identity/v3/xml/policy_client.py
+++ b/tempest/services/identity/v3/xml/policy_client.py
@@ -17,8 +17,8 @@
 
 from tempest.common import http
 from tempest.common import rest_client
+from tempest.common import xml_utils as common
 from tempest import config
-from tempest.services.compute.xml import common
 
 CONF = config.CONF