Bump yangtools to 3.0.0

Bump yangtools to 3.0.0

This bumps yangtools to 3.0.0, adjusting by API changes introduced
in this release:
- EffectiveModelContextFactory is required by yang.model.repo.api
  constructs
- JSONParserStream.create() deprecated methods removed
- ParserConfigurationException no longer thrown from UntrustedXML
- strict if-feature validation by YANG parser
- SchemaUtils.getNodeIdentifierForAugmentation() removal
- LeafrefSchemaNode correctly using PathExpression and not the
  overly-general RevisionAwareXPath
- NormalizedNodeStreamWriter updates
- NormalizedNodes no longer having attributes

The last two items require a complete overhaul of how edit operation
is disassembled in EditOperationNormalizedNodeStreamWriter. This
is addressed in a follow-up patch.

Change-Id: Ic58c7392d6326bc785bc2f1da9f5dee866e830dc
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>