support set Interface w/ external-ids

This is a **Feature Request**

ovs-vsctl is able to set external-ids properties to records in ovsdb
```
ovs-vsctl set Interface foo external-ids:iface-id=bar external-ids:iface-status=active external-ids:attached-mac= external-ids:vm-uuid:
```
This is actually used by OpenStack Nova agent to pass information for Neutron agent to populate proper overflow rules.

go-openvswicth has (v *VSwitchSetService) Interface(ifi string, options InterfaceOptions) method able to set Interface properties, however, not external-ids ones.

go-openvswitch should extend the Interface method to make it possible to set external-ids key-value pairs.