Fixes for S3 plugin
Fixes for S3 plugin

- Handle vlaidated default for skip_ssl_validation
- Default s3_host to s3.amazonaws.com
- Default signature_version to 4
- Default prefix to "" (backups in root of bucket)

Fixes #157 and #158

starkandwayne/shield,karampok/shield,starkandwayne/shield,karampok/shield,starkandwayne/shield,starkandwayne/shield,starkandwayne/shield,karampok/shield,karampok/shield