AWS_SECURITY_TOKEN support

[jguzman-splunk]

Hi @mishudark ,

I've added an if/else clause to support connecting with tokens, this allows s3-parallel-put to work with AWS setups using Okta IDP, which supports authentication only via ephemeral token auth. If no session token variable is present, it will create the connection with the id/secret credentials as usual.

PR here: #55