specifying default env var values

[viktorbenei]

it's usually a good idea to define default values

1. for inputs which should be specified in Secrets, instead of directly for the input
2. for inputs which will be most likely reused in another step, so it's enough to define it once, either as an App Env Var or Secret

[fadookie]

@viktorbenei looks fine. this was forked from https://github.com/bitrise-io/steps-amazon-s3-upload so you should also apply this fix there