S3 provider doesn't work with MFA credentials

[eschrock]

Issues Addressed

Fixes #30

Proposed Changes

This fix will fetch the session token from the client and pass that to the server. Why you have to check the instance type of the object returned I don't know, but that's apparently the interface.

Testing

After getting credentials from aws sts get-session-token and setting environment variables, with current titan:

$ titan clone s3://titan-data-demo/hello-world/postgres hello-world
Clone failed.
The AWS Access Key Id you provided does not exist in our records. (Service: Amazon S3; Status Code: 403; Error Code: InvalidAccessKeyId; Request ID: AE9AE0C236B7E357; S3 Extended Request ID: uKxN8c7SyScgymGk4i3BhzY9LMRcxCNJb4UFn9UeX3BVgRrCVXXGxNzd+vSXr0zwMAF7uXwFPyc=)
hello-world removed

With fix in place:

$ java -jar ./target/titan-0.3.1-jar-with-dependencies.jar clone s3://titan-data-demo/hello-world/postgres hello-world
Creating repository hello-world
Creating docker volume hello-world/v0 with path /var/lib/postgresql/data
Running controlled container hello-world
PULL 0f53a6a4-90ff-4f8c-843a-a6cce36f4f4f from origin RUNNING
Pulling 0f53a6a4-90ff-4f8c-843a-a6cce36f4f4f from 'origin'
Downloading archive for /var/lib/postgresql/data
null
Extracting archive for /var/lib/postgresql/data
null
PULL 0f53a6a4-90ff-4f8c-843a-a6cce36f4f4f from origin COMPLETE
Stopping container hello-world
Checkout 0f53a6a4-90ff-4f8c-843a-a6cce36f4f4f
Starting container hello-world
0f53a6a4-90ff-4f8c-843a-a6cce36f4f4f checked out