Interface driver support for Indigo Virtual Switch (IVS)

Indigo Virtual Switch (IVS) is a pure OpenFlow virtual switch designed for high performance and minimal administration. It is built on the Indigo platform, which provides a common core for many physical and virtual switches. More details on IVS can be found here:
https://github.com/floodlight/ivs/blob/master/README.md

This blueprint will introduce quantum interface driver support for plugging an interface into IVS.

Scope: Add IVS VIF type to Quantum to allow plugins to reference the type on Nova compute nodes. Also adds IVS interface class to allow agents (e.g. DHCP) to bind to an IVS switch.

Use Cases: Support the use of the Indigo virtual switch in place of open vSwitch.

Implementation Overview: Add an interface class for the Linux network agent. Add the new VIF type for use by the plugins.

Data Model Changes: Adding new interface class for agents and a new VIF type constant.

Configuration variables: Agents can bind to the interface by referencing the class. Plugins can use the new VIF type in their port bindings.

API's: N/A

Plugin Interface: Plugins can reference the new VIF type in their portbindings.

Required Plugin support: N/A

Dependencies: Nova compute nodes will need to have IVS installed.

CLI Requirements: N/A

Horizon Requirements: N/A

Usage Example: To use it with the BigSwitch plugin, set the vif_type to 'ivs' in etc/quantum/plugins/bigswitch/restproxy.ini

Test Cases: unit tests are included in the code.