Fixes NeutronPort.device_owner proto definition.

Neutron's Port object's "device_owner" attribute is an enum field, but
corresponding NeutronPort's device_owner is a string field even though
DeviceOwner enum has been defined. Changed the proto field to an enum
field.

Change-Id: Id01afa38bb52fec729219aa6c1d2e629e4fc1ba0
Merge "Fixes NeutronPort.device_owner proto definition."