API: Add support for "defaults"

API: Add support for "defaults"

Add support in the API language for specifying a field default.
Add default support in Python binding.

define foo {
  u16 mtu [default = 1500];
};

This is client side only. I.e. if the mtu argument is not passed to the foo function,
the client language binding will set it default to 1500.

Change-Id: I5df43f3cd87cb300b40ca38e15dcab25b40e424a
Signed-off-by: Ole Troan <ot@cisco.com>