Mark floating-IP auto-assignment as disabled also with Neutron

Neutron does not implement the feature right now, so using Neutron
is like using Nove-Network with auto-assignment disabled.

See the Neutron spec at:
https://blueprints.launchpad.net/neutron/+spec/auto-associate-floating-ip

Change-Id: If8bbd8adb69bf03649f6c61a6bc9f560a9913fbc