VPNaaS integration with service type framework

* adds new attribute 'provider' for vpnservice
* adds support for multiple drivers for vpn

Implements blueprint vpn-service-types-integration
DocImpact

Change-Id: I9e195dfaee21b1cf204cb9b9fc773bc9e5af5936
VPNaaS Service Driver for Cisco CSR

This has the service driver part of the vendor specific VPNaaS plugin.
The changes rely on the Service Type Framework code, which is presently
under review (client 53602, server 41827). The device driver will be
under a separate review.

Note: The Cisco plugin will require an out-of-band CSR setup and
      running for operation.

Rev 2 has fixes for tox errors.
Rev 3 has rebase to get latest 41827 changes.
Rev 4 has code review comments, moving vendor config file chgs to 74156
      review.

Change-Id: I39b1475c992b594256f5a28be0caa1ee9398050e
Partially-implements: blueprint vpnaas-cisco-driver