Add partial specs support in ML2 for gre/vxlan provider networks

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

In practice, for GRE/VXLAN provider networks provider:segmentation_id
choice can be delegated to neutron, in such case neutron will try to
find a network in tenant network pools which respects provided
provider attributes.

DocImpact

Related to blueprint provider-network-partial-specs
Partial-Bug: #1330562

Change-Id: I720d7d04f6e3453145e888d9e4d9b5e381d0f7d4
Merge "Add partial specs support in ML2 for gre/vxlan provider networks"