Add "--network-segment" option to the "subnet create" command.
This is a beta command option and subject to change. Use global
option "--os-beta-command" to enable this option.
This patch set also provides a devref update for beta command
options.
Change-Id: I4d0fbe079b2a873307364c41c22ce9ba88e632e6
Partially-Implements: blueprint routed-networks