Avoid segment DB lookup in _expand_segment on port
Avoid segment DB lookup in _expand_segment on port

The port context already has a network attached to it that contains
all of the segments for that network. In all cases the segments the
port wants to lookup should be present here so we can just iterate
over them without a separate DB query.

Partial-Bug: #1665215
Change-Id: I9014c4e401134f4db908ed989e0095d0ca95b7c9

openstack/neutron,mahak/neutron,eayunstack/neutron,openstack/neutron,noironetworks/neutron,eayunstack/neutron,huntxu/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,huntxu/neutron,mahak/neutron