New default-system statement to indicate that the default value is not constant (determined by the system)

[reshad-equinix]

See #112 which we agreed is of low-importance.

When the default is determined by the system (conditions usually mentioned in the description), it'd be handy for tooling to know that such is the case. e.g. if a node is not mandatory and doesn't have a default, this is usually flagged as a warning, but with the "default-system" statement that wouldn't be the case.

[BalazsLengyel]

Support this. We have a need for this.

[janlindblad]

Should it be a "default-by system" vs "default-by model" kind of thing, to make it a bit similar to ordered-by?

[QiufangMa]

I think this is useful, to indicate that some node, if not configured explicitly, will be allocated a value by the system.

[kwatsen]

Could this be solved a different way, by letting the <system> datastore set the leaf's value?

[BalazsLengyel]

Our solution:

extension default-system-created {
description
"Indicates that if a value is not set explicitly when the parent
container, or list is created the system will set the value.
The statement can be used instead of the YANG default statement when the
value is not set to a specific fixed value, but to a value that is not
known at design time or a value that may be different on subsequent
occasions.

   The value is set
   - when the leaf/leaf-list's closest ancestor node in the schema tree
   that is not a non-presence container is created
   ~ an ancestor list entry or presence container is created
   ~ an ancestor case statement becomes selected explicitly or as a
   default case
   ~ a when statement for this list or its ancestor statement becomes true
   - the server starts supporting a new YANG module with a top level
   leaf/leaf-list

   The parent leaf/leaf-list's description statement SHOULD explain how the
   value is generated.

   The statement MUST only be a substatement of a leaf or leaf-list statement
   that represents configuration data. The parent leaf/leaf-list MUST NOT
   have a mandatory, min-elements or default substatement.
   Zero or one default-system-created statement is allowed per parent
   statement.
   No substatements are allowed.

   Adding or removing default-system-created is a backwards-compatible
   change.";

}

[kwatsen]

Andy: we call it a "dynamic default".
Qiufang: this issue relates to creating a list item where the server can fill in a system value when the item is created.
Reshad: like when configuring an interface, and device fills in some values, may or may not depend on <system>

[abierman]

Our server follows the same behavior described by Balazs extension.
Key point in conf-call: the default value may depend on internal state info at that moment,
which may not be representable as instances of the corresponding YANG objects.