make Redshift sslmode configurable
make Redshift sslmode configurable

This makes it possible to configure Redash to require TLS
and verify the certificate Redshift presents.

The setting works similarly to that of the PostgreSQL source
except that Redshift explicitly passes 'prefer' to psycopg2
if sslmode is not configured (for compatibility).

chriszs/redash,moritz9/redash,alexanderlz/redash,chriszs/redash,chriszs/redash,44px/redash,moritz9/redash,alexanderlz/redash,hudl/redash,hudl/redash,denisov-vlad/redash,44px/redash,alexanderlz/redash,getredash/redash,getredash/redash,getredash/redash,moritz9/redash,getredash/redash,denisov-vlad/redash,hudl/redash,denisov-vlad/redash,hudl/redash,44px/redash,moritz9/redash,getredash/redash,44px/redash,denisov-vlad/redash,denisov-vlad/redash,alexanderlz/redash,chriszs/redash