Changed usage of the CREDENTIALS command for Redshift to specific
keywords. In other words, the commands produced will no longer be
of the form:
CREDENTIALS('access_key=X&secrety_key=Y')
and will instead be of the form
access_key = 'X' secret_key = 'Y'

This is needed because for loading encrypted payloads into Redshift using
client side encryption, one
needs to place symmetric_master_key as an argument on the copy command, however,
it is also an options within the CREDENTIALS command, so if a query to Redshift
includes a CREDENTIALS clause and also symmetric_master_key, then Redshift will
report this error:
com.amazon.support.exceptions.ErrorException: Amazon Invalid operation: conflicting or redundant options;

When data is encrypted in S3 and a COPY command is invoked, it's expected
that the manifest is not encrypted and is in plain-text form.

Encryption on EMR through EMRFS is controlled by a Hadoop option (fs.enable.cse).
Once set, all data that goes through the file system will be encrypted.

This commit adds an exception around generation of manifest files so that even
if the encryption option is set, the manifest file created on S3 is not encrypted.

This enables Redshift to read the manifest and ingest the data even for cases where
data is encrypted on the client side with a symmetric encryption key.

Switching the UNLOAD statement to no longer use the WITH CREDENTIALS
method and instead rely on explicitly passing the role, access key,
secret key, session token, etc.

Generally speaking this is a more flexible way of passing credentials,
though for UNLOAD it doesn't make much difference. This change is
pursued to achieve consistency with the COPY command. In COPY command,
this change is necessary to enable copy of client-side encrypted
data with Redshift.

http://docs.aws.amazon.com/redshift/latest/dg/copy-parameters-authorization.html