Support Databricks unity catalog

[NiallRees]

Currently, the package assumes that the database cannot be set for the Databricks adapter. In the newly released Unity Catalog however, there is now the ability to create multiple catalogs per account, which are the equivalent of a database (see databricks/dbt-databricks#95).

This issue will be closed by a pull request which adds support for Unity Catalog, meaning that users can specify both their raw source tables and models to be created in a catalog of their choosing.