Fix: snapshots should create relations with database=none

[jtcohen6]

Problem

The snapshot materialization twice needs to get or make a relation. Previously, it set the database by grabbing the value of the existing/model relation's database. This resulting in an error:

hologram.ValidationError: Undefined is not of type 'string'

Failed validating 'type' in schema[0]:
    {'type': 'string'}

On instance:
    Undefined

Solution

We should explicitly set database=none, in line with the changes in dbt-spark==0.17.0 that preclude database from having a value.

Note on testing

I caught this in my local end-to-end testing of all current features. This wasn't caught in automated integration testing because:

• dbt-integration-tests suite does not include a dbt snapshot step
• snapshots are a Delta-only features, and current integration tests run only on dockerized Apache Spark

The resolution involves:

• better testing for plugins (Test suite for non-core plugins dbt-core#2492)
• an additional set of tests for Delta-only features, on a Databricks runtime, that could run in parallel with standard feature testing on dockerized spark (Add integration tests for Delta-specific features #86)