Add partial specs support in ML2 for multiprovider extension

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