fix credentials references for uploading to redshift

aws metadata iam credentials are sts tokens. you have to include the token with the credentials passed to redshift as described here: http://docs.aws.amazon.com/redshift/latest/dg/r_copy-temporary-security-credentials.html

for the upload to s3 portion, not specifying credentials means that boto will automatically use the iam role's credentials. not sure if this will work, but I think we should try it.