Upcoming Duo changes for Argon

As part of the Summer 2021 HPC maintenance on August 11, 2021, there will be a change made to the implementation of Duo on the backend. This will provide a slightly higher level of security and address an inconsistency in behavior for some ssh related commands. There are some commands, such as scp, sftp, and even FastX, that always send a Duo auto-push. In these cases, there is no opportunity to use a Duo key, which is inconsistent with how Duo works with other services, including ssh itself. The reason for this behavior is historical and was necessary at one time, but that is no longer the case. As part of this change, the DUO_PASSCODE environment variable will no longer be processed by the SSH service for SSH login sessions on Argon. While this will be a loss of that feature, we feel that the increased security, the ability to use tokens in all cases, and consistency in behavior is the proper path forward.