devstack: use stevedore entry point for flavor service plugin
devstack: use stevedore entry point for flavor service plugin

It's discouraged to use python import paths. Instead, we should use
stevedore entry points.

Change-Id: I7e3fce5e63ad309d761d9fec713a31fa33966eef

cloudbase/neutron,openstack/neutron,bigswitch/neutron,sebrandon1/neutron,noironetworks/neutron,sebrandon1/neutron,mahak/neutron,cloudbase/neutron,igor-toga/local-snat,mahak/neutron,dims/neutron,dims/neutron,igor-toga/local-snat,MaximNevrov/neutron,wolverineav/neutron,klmitch/neutron,eayunstack/neutron,openstack/neutron,MaximNevrov/neutron,openstack/neutron,noironetworks/neutron,bigswitch/neutron,wolverineav/neutron,eayunstack/neutron,klmitch/neutron,mahak/neutron,huntxu/neutron,huntxu/neutron