Generate catalog for relational, non-ephemeral nodes only

[jtcohen6]

Similar to what #3716 did for schema caching, we should do for cataloging, too. If we expect a node to exist as an object in the database, add its database-schema to the set of namespaces to catalog; if it's ephemeral or otherwise non-relational, ignore it.

I think this would be as simple as updating the logic in _get_catalog_schemas to check for node.is_relational and not node.is_ephemeral_model, as we do in _get_cache_schemas:

https://github.com/dbt-labs/dbt/blob/05dea18b625253625c135cbf37311d0206a8cc79/core/dbt/adapters/base/impl.py#L316-L337