Merge "Add IPDevice.exists() method"

diff --cc neutron/agent/linux/ip_lib.py
Simple merge

diff --cc neutron/plugins/ml2/drivers/linuxbridge/agent/linuxbridge_neutron_agent.py
Simple merge

diff --cc neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py
Simple merge

diff --cc neutron/tests/unit/agent/linux/test_ip_lib.py
Simple merge