JCLOUDS-635 Always check for container existence before uploading blobs.

This guarantees that the same error code is returned for all cloud providers
if the container does not exist.

Fixes #635.
This is a big hammer to solve a small problem: all write commands will require two RPCs even in the case of when the container exists and for non-Swift blobstores.  Can we do something better than this, possibly in the Swift provider itself?