Fix mandatory enforcer failure on augmented nodes

Fix mandatory enforcer failure on augmented nodes

Changes the way mandatory nodes are extracted from the schema.
Previously all mandatory nodes were approached as direct children of
the parent node. Even if they came from an augmentation which caused
"missing mandatory descendant" exceptions.
This patch preserves both options of building data nodes containing
augmented mandatory nodes.
- setting the mandatory node as a direct child of the parent node
- setting the mandatory node as an AugmentationNode to the parent node

However the former option seems to be incorrect and might be removed
in the future major release.

JIRA: YANGTOOLS-1276
Change-Id: Ifcc2e0d141e18f74173f15e293b16cbd297a5af6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Tibor Král <tibor.kral@pantheon.tech>