Switch pipeline secrets to only use credhub

[cweibel]

Changes proposed in this pull request:

• Switch App-AutoScaler pipeline secrets to only use credhub, removing the secrets file from s3
• Part of https://github.com/cloud-gov/private/issues/618

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

One less set of creds managed in s3, a bunch more to go...

[markdboyd]

I assume the full name of the credhub variable is longer, but perhaps not safe to document here in the repo?

[cweibel]

The names of all the credhub variables, along with relative or absolute paths, are already available in all of our manifests and ops files.

Good eye though, that's why I didn't provide the credhub cli command to update it