Modular L2 Multi-Segment Network API

The providernet extension currently exposes details of a virtual network using a (network_type, physical_network, segmentation_id) tuple of attributes. The ml2 plugin's DB schema and driver APIs support virtual L2 networks made up of multiple segments with different details. The network_type value "multi-segment" is used when network is made up of more than one segment, but there is currently no API for accessing the tuples describing these individual segments, nor for adding segments to a network to form a multi-segment network. A separate resource type, either top-level or as a sub-resource of network, is needed, with the existing provider attributes on network retained for compatibility and ease-of-use for single-segment networks.