OS 523 Create entry point for ipam_driver in setup.cfg

Update ipam_driver load workflow to use entry points in setup.cfg.
Reflect ipam_driver option in neutron.conf.
FYI, setup.cfg values are cached, so changes in this file are not seen on local machine unless you run
'python ./setup.py install' to populate changes from setup.cfg.
You should provide some more realistic value here, e.g. `ipam_driver = neutron.ipam.drivers.neutron_ipam.NeutronIPAM`