VPNaaS integration with services flavor framework

https://review.openstack.org/457527
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 058469e1b99b647537a5228c6a384d93df5484df
Author: Hunt Xu <email address hidden>
Date: Wed Apr 12 19:20:43 2017 +0800

    VPNaaS integration with services flavor framework

    * adds new attribute 'flavor_id' for vpnservice
    * adds support for multiple drivers for VPNaaS

    This solution uses a flavor of service type VPN associated with a flavor
    profile containing a driver. During vpnservice creation, if a flavor_id
    is passed, it is used to find the provider for the driver which would
    handle the newly created vpnservice. The flavor_id and the
    provider-vpnservice association are pesisted in the DB.

    ApiImpact and DocImpact tags are added as the new optional flavor_id
    parameter should be described, as well as the support of multiple VPNaaS
    drivers.

    The original work and discussion about integrating VPNaaS and service
    type framework can be found in the following change:
    I9e195dfaee21b1cf204cb9b9fc773bc9e5af5936

    ApiImpact
    DocImpact
    Closes-Bug: #1672920

    Signed-off-by: Hunt Xu <email address hidden>

    Change-Id: I0095e160481f1d4572e38ad1d3bbc8e183039b84