VPNaaS refactor service driver to reuse VpnDriver code

[Sridhar Ramaswamy]

Currently various VPN service-drivers in neutron-vpnaas repo doesn't reuse code which is same for most part. Topic connection creation and make_vpnservice_dict (again optional) are different for each service-driver. This came up during the code review of [1].

The proposal is to refactor this code to have a base class with a separate "create_rpc_connection" method which will be overridden by different service-driver implementations.

[1] https://review.openstack.org/#/c/154720/5/neutron_vpnaas/services/vpn/service_drivers/vyatta_ipsec.py,unified

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/158516
Committed: https://git.openstack.org/cgit/openstack/neutron-vpnaas/commit/?id=62dffba3d75376a91f2b971f571cd749c04a78a5
Submitter: Jenkins
Branch: master

commit 62dffba3d75376a91f2b971f571cd749c04a78a5
Author: Sridhar Ramaswamy <email address hidden>
Date: Mon Feb 23 15:20:32 2015 -0800

    VPNaaS refactor service driver to reuse VpnDriver code

    Introduce BaseIPsecVpnDriver class to host most of the
    common code to be shared across various implementations
    of VPN service-drivers. This reduces the duplicate code
    in these service-drivers

    Closes-Bug: #1422905
    Related-Bug: #1423244
    Co-Authored-By: Numan Siddique <email address hidden>
    Change-Id: Ibde249af5c7228e2f9b7496cb03bd9b516966ef5