Create temporary views with 'or replace'

[annazizian]

resolves #350

Problem

A re-run of an incremental model tries to create the temporary view <model_name>__dbt_tmp within the same session and fails. This happens when the view is not dropped in the previous run.

Solution

Just replace the view if it already exists. As the view is intended to be used within the run only, replacing the view on re-run won't affect anything and will solve the issue.

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

[colin-rogers-dbt]

closing as this was resolved in #906