Implementing XML client for VPNaaS

VPNaaS API test for xml request body is missing.
This blueproint implements xml client for VPNaaS testcases.

Change-Id: I236a889350e9b7d865e874b73f825b3a34809a3f
Implements: blueprint vpnaas-api-tests-xml
Merge "Implementing XML client for VPNaaS"