Issue #2332751 by tstoeckler, benjy: Added Allow to limit the nodes to migrate by node type.

core/modules/migrate/config/schema/migrate.source.schema.yml

...ules/migrate_drupal/src/Plugin/migrate/source/d6/Node.php

...te_drupal/tests/src/Unit/source/d6/NodeByNodeTypeTest.php

...l/tests/src/Unit/source/d6/NodeRevisionByNodeTypeTest.php

...rate_drupal/tests/src/Unit/source/d6/NodeRevisionTest.php

...ules/migrate_drupal/tests/src/Unit/source/d6/NodeTest.php

...ules/migrate_drupal/tests/src/Unit/source/d6/TestNode.php

...rate_drupal/tests/src/Unit/source/d6/TestNodeRevision.php