Fix wrong URLs in VPNaaS extension API

* GET /vpn/services/service-id -> GET /vpn/vpnservices/service-id
* /vpn/vpnservice/... -> /vpn/vpnservices/...
* /vpn/ipsec-site-connection -> /vpn/ipsec-site-connections

Closes-Bug: #1242751
Change-Id: Ifcdf3c2c7402210bfe8caabf4ad5c27d7c7b3f85
Merge "Fix wrong URLs in VPNaaS extension API"