converting readme vars to global vars

[fivetran-sheringuyen]

Are you a current Fivetran customer?

Fivetran employee

What change(s) does this PR introduce?

Updating vars to be global variables

Does this PR introduce a breaking change?

• [ ] Yes (please provide breaking change details below.)
• [x] No (please provide explanation how the change is non breaking below.) Just change to readme

Is this PR in response to a previously created Issue

• [x] Yes, Issue 57
• [ ] No

How did you test the PR changes?

- [x] CircleCi - [ ] Other (please provide additional testing details below) **Select which warehouse(s) were used to test the PR** - [x] BigQuery - [x] Redshift - [x] Snowflake - [x] Postgres - [x] Databricks - [ ] Other (provide details below) **Provide an emoji that best describes your current mood** :dancer: **Feedback** We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your [feedback](https://www.surveymonkey.com/r/DQ7K7WW) on our existing dbt packages or what you'd like to see next.