Added NormalizedNodeContainerBuilder and helpers.
- NormalizedNodeContainerBuilder is common root
to builders which produces NormalizedNodeContainer,
this allows to write more generic code using builders.
- NormalizedNodeUtils - provides helper functions to process
normalized date tree, such as findNode which look up
subtree based on provided InstanceIdentifier.
- Added Immutable marker interface to existing builders.
Change-Id: Id842742432c48d76cdf78dab40352af27e10360f
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>