rgw/sfs: expose 'S3GW_' prefixed seed credential env variables to user instead of 'RGW_' prefixed env vars

[jecluis]

Instead of exposing the user of s3gw to RGW_.* prefixed environment variables to set the seed access and secret keys, use S3GW_.* environment variables instead.

This can potentially be achieved by translating variable names in the wrapper script in #758 .