Re-use context session in ML2 DB get_port_binding_host

Re-use context session in ML2 DB get_port_binding_host

This patch modifies ML2 DB get_port_binding_host method so that it
reuses the existing context session to do the database query
rather than creating a new database session.

Note that there are other methods in ML2 DB that do not re-use
the caller's session (get_port_from_device_mac() and
get_sg_ids_grouped_by_port()). These will be modified using
a separate bug (https://bugs.launchpad.net/neutron/+bug/1441205).
Change-Id: I8aafb0a70f40f9306ccc366e5db6860c92c48cce
Closes-Bug: #1440183