VPNaaS Service Driver for Cisco CSR

This has the service driver part of the vendor specific VPNaaS plugin.
This version DOES NOT rely on the Service Type Framework code, which is
presently under review (client 53602, server 41827) and on hold due to
discussion over flavors. As a result, this changeset has modifications
so that the service driver is not hard-coded in the VPN plugin.

The device driver will be under a separate review and has the REST
client that talks to the Cisco CSR (running out-of-band).

Note: See review 74156 for more details on device driver portion of
      this blueprint.

Change-Id: I39b1475c992b594256f5a28be0caa1ee9398050e
Partially-implements: blueprint vpnaas-cisco-driver
Adds the missing migration for gw_ext_mode

Requires adding the plugin to the script for
db migration for the ext_gw_mode in routers table.

Change-Id: Iefa121b5c3a60ed804bf3927d91e64d843a28fd1
Closes-Bug: #1288420
Merge "VPNaaS Service Driver for Cisco CSR"
Merge "Adds the missing migration for gw_ext_mode"