Bug 5335: augmenting a mandatory node on a presence container

Yang parser didn't perform cross module augmentation of mandatory
node on a presence container which belongs to the same module.
RFC6020 section 3.1 considers a presence container as not mandatory
node, therefore such augmentation should be allowed.

Change-Id: I61f45519db3536ab377fdf2987b083806a3aa51e
Signed-off-by: Peter Kajsa <pkajsa@cisco.com>