In order to speed up containers creation/deletion a new generic-vif
driver is proposed that build upon the port pool driver to ensure
ports already created can be reused in the future.
Note this remove the neutron.create_port from the container creation process.
As measured in the performance evaluation performed in [0], these times are,
on average, around 2 seconds.
[0] https://blog.russellbryant.net/2016/12/19/comparing-openstack-neutron-ml2ovs-and-ovn-control-plane/
Partially Implements blueprint ports-pool
Change-Id: Ib127735570470850dde452c453eac3d5545f7a43
e01b4d559b