Allow S3 bucket to register to be specified on command line (PLAT-553).

This allows for credentials that have no LIST BUCKETS privileges to be passed to S3.
Fixed S3Api::Init() to work properly with credentials, environment and keys; make env have precedence over credentials (PLAT-553)
Merge pull request #90 from datacratic/PLAT-553-S3-bucket-command-line

Allow S3 bucket to register to be specified on command line (PLAT-553).