Redshift translator "AS decimal" should have 0 default scale

Currently "AS decimal" relies on the posgresql behavior of using a non-zero default scale: https://www.postgresql.org/docs/10/static/datatype-numeric.html
Whereas redshift has a 0 default: https://docs.aws.amazon.com/redshift/latest/dg/r_Numeric_types201.html

So this needs updated in the base postgresql translator to work correctly on redshift.