Redshift: add support for the new ACM root CA.

[arikfr]

Closes #1974.

[galgoogle]

@arikfr We've "fixed" it on our side by changing the pg.py file.
We've downloaded the certification file redshift-ca-bundle.crt.
And under /opt/redash/current/redash/query_runner we've edited pg.py:

if "redshift" in self.configuration.get('host'):
                connection = psycopg2.connect(user=self.configuration.get('user'),
                                              password=self.configuration.get('password'),
                                              host=self.configuration.get('host'),
                                              port=self.configuration.get('port'),
                                              dbname=self.configuration.get('dbname'),
                                              sslrootcert='/opt/redash/redshift-ca-bundle.crt',
                                              async=True)

That's because our hostname contains the string "redshift". I'm sure you can do it in a more elegant way.

[arikfr]

@galgoogle that's pretty much what this pull request does just in a cleaner way :)