VPN as a Service (VPNaaS) API and DataModel

implements:blueprint VPNaaS-Python-API

It provides the basic API and
DataModel for the "VPN As A
Service" feature that includes:
* VPNService
* IPsecSiteConnection
* IKEPolicy
* IPsecPolicy

Change-Id: I059d4db05118a4eecd388b8e8db0d714a8f9cb8f
Merge "VPN as a Service (VPNaaS) API and DataModel"