Add NeutronService, to fetch topology updates from Neutron

Not yet tested, as this will become much easier to test once the
Neutron team has their components ready to generate the test data. The
code isn't hooked up, so it shouldn't break anything.

Change-Id: I1489de59e7c12b2af2aacd3931404c224585b413
Signed-off-by: Brandon Berg <bberg@midokura.com>