Stow makes too many API, also fails to list buckets in other regions

[svyotov]

Issues detected in this section of the code:

• Stow makes too many API calls to AWS when listing buckets
• Stow assumes that the bucket region filter is the same with the authentication region (does not return the full list of buckets, but this is a bug, because if the region is not specified at all stow fails to authenticate to AWS, which means the user cannot retrieve all the buckets )
• Over complicated code, that probably should be moved to a function. This is also duplicated in the Container(..) function.
  https://github.com/graymeta/stow/blob/926777b8f7aaf9b57d475c612baebb0968532c48/s3/location.go#L104-L125