Midonet plugin support for Neutron

Initial proposal to add Midonet's plugin support for Neutron, so the complete midonet solution can be deployed with ansible-openstack project.

MidoNet is an open-source network virtualization platform for Infrastructure-as-a-Service (IaaS) clouds. MidoNet integrates with OpenStack's Neutron. As of the Liberty release, the following features are supported:

    Basic L2 (Network, Port and Subnet/DHCP)
    Metadata Service
    Router
    Security Groups and Rules
    Gateway (External Network/SNAT)
    Floating IP
    Agent/Agent Scheduler (DHCP agent only, used for Metadata Service)
    Extra DHCP Options
    Quotas
    Subnet Allocation
    LBaaS (Load Balancer as a Service)
    Docker with OpenStack
    FWaaS
    Port-security
    Allowed address pairs
    Extra Routes
    ML2 (separate, experimental driver)