e2e: move Sauce credentials out of source code (part 1)

Move SauceLabs credentials to credentials.js, and establish means
to read from that file or environment variables.

TODO: .gitignore credentials.js in a part 2.
e2e: move Sauce credentials out of source code (part 2)

.gitignore credentials.js so that modifications to the file are not
pushed/pulled into version control.