Revisit loading credentials from ENV once runner `cmd`'s are more clear.

[zjrgov]

This issue is just a reminder to revisit the way credentials are loaded in cf-executor/cg/creds.go. They currently load straight from the OS environment but we might want to move that once it's more clear how everything will be instantiated.

[rahearn]

This is likely to be the best/most straight forward way to do it moving forward as well, but I'm good with leaving this open and revisiting once the architecture is more settled