Missing max_routes in neutron.conf

We have the define of "max_routes" in neutron/db/extraroute_db.py
but we do not have them in neutron.conf.

Change-Id: I4a9586023b7d09a64296b44f4d9285f30be1fd63
Closes-Bug: #1341378
Merge "Missing max_routes in neutron.conf"