Add partial specs support in ML2 for vlan provider networks

Add partial specs support in ML2 for vlan 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 VLAN provider networks.

In practice, for VLAN provider networks provider:physical_network
and provider:segmentation_id choices 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: I2c52c71167edaa153b2e04681273e2f1be8d03aa