Add command-line flags to set S3 access configurations

It's annoying that the only way to manage S3 credentials right now is to put them in plain-text in core-site.xml. Instead, we should add command line flags to set fs.s3a.access.key and fs.s3a.secret.key directly, using hdfsBuilderConfSetStr().

The secret key at least should be expressed as a command to run which will yield the secret key (just like --pem_password_cmd).