sysconfig render does not support type manual subnets

Attempting to a render a network configuration like this:

% cat manual_subnet.yaml
network:
    version: 1
    config:
        # Physical interfaces.
        - type: physical
          name: interface0
          mac_address: "52:54:00:12:34:00"
          subnets:
              - type: manual
                address: 10.0.2.100/24

Attempting to render this into sysconfig results in a traceback.
% PYTHONPATH=`pwd` ./tools/net-convert.py --network-data manual_subnet.yaml --kind yaml --output-kind sysconfig -d target

Traceback (most recent call last):
  File "./tools/net-convert.py", line 82, in <module>
    main()
  File "./tools/net-convert.py", line 78, in main
    r.render_network_state(ns, target=args.directory)
  File "/home/rharper/work/git/cloud-init/cloudinit/net/sysconfig.py", line 410, in render_network_state
    network_state).items():
  File "/home/rharper/work/git/cloud-init/cloudinit/net/sysconfig.py", line 391, in _render_sysconfig
    cls._render_physical_interfaces(network_state, iface_contents)
  File "/home/rharper/work/git/cloud-init/cloudinit/net/sysconfig.py", line 311, in _render_physical_interfaces
    cls._render_subnet(iface_cfg, route_cfg, iface_subnets[0])
  File "/home/rharper/work/git/cloud-init/cloudinit/net/sysconfig.py", line 238, in _render_subnet
    iface_cfg.name))
ValueError: Unknown subnet type 'manual' found for interface 'interface0'

Sysconfig does have an equivalent of eni's 'manual' control mode, the ONBOOT=N setting tells sysconfig to ignore this interface when booting. Later users can ifup the interface.

1. no cloud, just in-tree rendering
2. cloud-init master branch