Read env variables for exporter keys

[critzo]

Currently exporters read a keyfile that is packaged with a release on each device. Replace this by having each exporter read the contents of a key from an environment variable.

Relates to GCS, SCP exporters.

[critzo]

Per @robertodauria's initial research, this requires keys to have no newlines and be converted to base64 encoding. We can probably put together a script that does this, and add it to utilities/, complemented with documentation.