BUG 8566 Introduce a fallback for ChoiceSchemaNode lookup

With the way parsing callbacks are handled between netconf and yangtools
theres no hook for case nodes, which we are missing in the path when doing
SchemaNode lookup. This introduces a callback that checks Choice children
for a possible match once the normal lookup fails.

Change-Id: Ieefc6771ec3c6892e00103cbab77f6340f197d62
Signed-off-by: Tomas Cere <tcere@cisco.com>