Update pipeline to use credhub vars

[markdboyd]

Changes proposed in this pull request:

• Deprecate use of S3 credentials file in favor of Credhub

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

Credhub is the authorized secret store for Concourse, so updating the pipeline to use Credhub for variables instead of a file improves security and consistency across our pipelines