Move fake_network config option to linux_net

Option 'fake_network' is used by nova/network/linux_net.py but not
imported from nova/network/manager.py. Both use it, but manager imports
linux_net (via nova/network/api.py + nova/network/floating_ips.py) so it
makes more sense to define the option in linux_net and import_opt in
manager.

Precise example import chain:
    bin/nova-manage
    nova/cmd/manage.py: import_opt('flat_network_bridge', 'nova.network.manager')
    nova/network/manager.py: from nova.network import api as network_api
      -- At this point, manager has started to import but not yet called
      register_opts
    nova/network/api.py: from nova.network import floating_ips
    nova/nova/network/floating_ips.py: import_opt('public_interface', 'nova.network.linux_net')
    nova/nova/network/linux_net.py: import_opt('fake_network', 'nova.network.manager')
      -- BANG! Manager hasn't registered the option we want to import

Change-Id: I66f021b34aef650dca148a58203c249f6aa0e83e
Merge "Move fake_network config option to linux_net"