Resolves #1897

Both settings will modify resulting S3 cache URL as pointed out by @ayufan in #1897:

- By default, we keep current behavior, URLs have the form /runner/<token>/project/<id>/<cache-key>
- When shared flag is enabled, the runner segment is removed from the URL: /project/<id>/<cache-key>
- In any case, we could add a Path prefix to that URL: <path>/runner/<token>/project/<id>/<cache-key>

The modifications made are:

- Modified getCacheObjectName to prepend the path when setting is present
- Added tests to verify previous behaviour of getCacheObjectName, added
  new ones to make sure feature works properly
- Added small paragraph in autoscaling feature to explain both settings