Allow returning data for successful <edit-config> operations beyond just <ok>

[BalazsLengyel]

In some cases an operation might have side effects. It would be good if these could be communicated in the message. Examples include:

• a leaf has a complex default value: not a specific default that can be documented in as a literal, but a value that is dependent on other conditions. It would be good to return the default value set.
• results in a background process that returns a value

[abierman]

Support this functionality, but it needs to be uniform for all operations, not just 'edit-config'.

In fact YANG already supports this feature using augment. Any RPC or action, whether it returns ok or data, can be augmented. A proper NC client will accept the augmented data instead of 'ok'