use rpc Connection rather than create_connection
use rpc Connection rather than create_connection

The create_connection function in neutron.common.rpc is just an alias to
Connection. This patch switches to the later so we can remove the former
in: https://review.openstack.org/#/c/560130

Change-Id: I60514447afcc2423821da4b5762f55056177874f

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow