Fix relative xPath resolution for leafref types

Fix relative xPath resolution for leafref types

If we have a leaf of type leafref with relative path that crosses
through case and choice schema nodes.

This stems from the historic disconnect about how the path should
be handled: it needs to be resolved in the context of walking the
data tree, not schema tree. That means that choice/case nodes are
not part of the path in the expression and therefore need to be
skipped over.

JIRA: YANGTOOLS-1100
Change-Id: I9908c91b0068f52f4b6719545c738d7c39e2a228
Signed-off-by: miroslav.kovac <miroslav.kovac@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>