Add support for hierarchical default values

[rgwilton]

Hierarchical configuration is often expressed in YANG models.

In this model, only the "default" statement should be placed at the root of the hierarchical configuration, and all other nodes in the configuration hierarchy must not have a default statement, and instead are required to state in text that the default value is inherited from the next node up in the hierarchical config chain.

Rather than expressing this default relationship in the description statement, it may be helpful to expressed this in code, via allowing the default value to be expressed using an xpath expression instead.

[abierman]

Do not support. This seems much more complicated to read and write for users. This makes it difficult to have descendant nodes from groupings. The defaults for the leafs should be where the leafs are defined.

Not Now: complexity: high, bc: unknown, importance: low