NETCONF southbound requires notifications.yang model to be present on the device

As noted in http://www.tail-f.com/integrating-confd-opendaylight/, our SB plugin requires notifications.yang to be present on the NETCONF server for notifications to work.

There is no standardized YANG model for NETCONF notifications simply because RFC5277 predates YANG and has not been updated.

As correctly mentioned in the note, though, the presence of the notification capability should be enough for notifications to work – the SchemaContext assembly code should supply the model internally based on the capability.