Merge "Configurable external gateway modes"

diff --cc quantum/db/l3_db.py
Simple merge

diff --cc quantum/extensions/l3.py
Simple merge

diff --cc quantum/plugins/openvswitch/ovs_quantum_plugin.py
Simple merge

diff --cc quantum/plugins/ryu/ryu_quantum_plugin.py
Simple merge

diff --cc quantum/tests/unit/test_db_plugin.py
Simple merge