Overwrite all rows from the existing seed tables and replace values

[machov]

resolves #112

Problem

The current seed command in dbt-spark appends to existing seeded tables instead overwriting when the database is 'unmanaged' (no location specified from create schema)

Solution

Overwrite all rows from the existing seed tables and replace values.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX