Drone S3 Region Support

[captn3m0]

Might need changes before merging back.

[donny-dont]

@Hashfyre @captn3m0 will one of you be addressing the feedback here?

[Hashfyre]

Hi @donny-dont I had first tried the region param passing to NewWithCredentials here:

• https://github.com/Hashfyre/drone-s3-cache/commit/7f02cbfa6b909f767de4c245d26cac1bdc505f9d

However that did not work for us, we had the same failed builds.

I'll try this again, and give you a detailed trace if possible.

Totally agree that not looking up creds through the *Creds object is a hack, will fix all of that before making a clean PR.