Fix yang-data extension definition

Fix yang-data extension definition

The definition on ietf-restconf's yang-data is being rather exploited in
the wild, with a license to contain choices and anydata. These are
essentially allowed by other tooling, hence we need to also relax our
requirements.

This implies we cannot really expose the container directly, but rather
have to implement DataNodeContainer instead -- with the implied API
breakage. This is fine due to @Beta-level contract of all interfaces
involved.

Since we are breaking semantics here, also ditch the notion that the
yang-data argument can be parsed into a QName and instead reuse the
schema tree child's QName as the value required by SchemaNode contract.

JIRA: YANGTOOLS-1443
Change-Id: I3f78f71ea2980bf84f409d1ec89979e9e7b200ae
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>