All of the externally consumed variables from neutron.common.constants
now live in neutron-lib. This patch removes neutron.common.constants
and switches all uses over to lib.
NeutronLibImpact
Depends-On: https://review.openstack.org/#/c/647836/
Change-Id: I3c2f28ecd18996a1cee1ae3af399166defe9da87