Add partial specs support in ML2 for multiprovider extension

[OpenStack Infra]

https://review.openstack.org/101467
commit 926d8a939ab97d0eb41b04d495f865a64da11b43
Author: Cedric Brandily <email address hidden>
Date: Fri Jun 20 11:31:01 2014 +0200

    Add partial specs support in ML2 for multiprovider extension

    ML2 provider network partial specs let admins choose some
    multiprovider network attributes and let neutron choose remaining
    attributes. This change provides the implementation for
    multiprovider networks.

    In practice, for VLAN/GRE/VXLAN segments provider:segmentation_id
    choice can be delegated to neutron, in such case neutron try to find
    a segment in tenant network pools which respects provided segment
    attributes. For VLAN segments provider:physical_network choice can
    also be delegated.

    DocImpact

    Implements blueprint provider-network-partial-specs

    Change-Id: I1cf1441a179ec527674276b71e9924841f8570b6