Add db_instance_type and db_engine_version to main variables

[wrbooth]

Adding the ability to change db_instance_type and db_engine_version from the main metaflow terraform module.

Testing Screenshots

RDS micro instance with engine version 14

[casperbiering]

@oavdeev Do you have time to take a look at this please?

[oavdeev]

Sorry it took so long, we'll merge & test & cut a new release in the next few days

[JPWallhorn]

@oavdeev Have you had a chance to look at this? This change would be very helpful for us too. Thank you.