Enforce augment statement argument well-formedness

Enforce augment statement argument well-formedness

'augment' statement has dual use, depending on where it is declared.
Each use requires a different schema node identifier:
- plain augment requires SchemaNodeIdentifier.Absolute
- uses/augment requires SchemaNodeIdentifier.Descendant

Make sure we enforce proper parsing, so that our argument will end
up being correctly captured (and can be used for further reasoning).

JIRA: YANGTOOLS-1189
Change-Id: I4e628fd1aee29e103a045cfe90bae58f91ce32ed
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>