Wider S3 deployment options

[9gel]

• Set default S3 URL to None for boto, so the library can construct the URL from the region, thereby allowing intra-VPC S3 access to work instead of always relying on public URL
• Set encryption to updated s3v4
• Use get bucket location as a test for connectivity instead of creating bucket, which work work well when there are a lot of workers starting at the same time
• Better error message when connectivity failed