[#690] Authenticated interaction with Amazon S3

Implements authenticated requests using Amazon S3 REST api

* Backward compatibility:
  * Devices running `1.x.x` APK expect that  `/surveys/*` is public. So survey definition will remain public
  * Devices running `1.x.x` APK will upload data to `/devicezip/*` and marked as `public-read`, when the server gets the notification for data ingestion, the first action is to mark the object as `private`
I probably missed it but wondering whether you took these changes into account --> https://github.com/akvo/akvo-flow/commit/fe382cfc0f0af31b93922801df15e0bac9832cdd. Although since its a utils method now it may cause problems with other use cases.