Add 'flat' network type to host config docs
Add 'flat' network type to host config docs

Flat network is not included in any of the hostconfig documentation, but
it is listed as an allowed configuration type in the python tool.

Change-Id: I3322a3763edcea60d6992196ed168aac8757af19
Signed-off-by: Tim Rozet <fa77065b01316237fb8724959b2d0a325cd92ce5@redhat.com>

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl