utils: don't nuke root dir in prep_container_storage()

[miabbott]

The function should just nuke the containers dir, rather than the
root of the containers dir.

[cgwalters]

/lgtm

[miabbott]

The aws-test job was failing because it couldn't find the JSON file it needed to copy to S3.

+ aws s3 cp --acl public-read /home/jenkins/workspace/coreos-rhcos-aws-test/aws-us-east-1.json s3://****/aws-us-east-1-tested.json

The user-provided path /home/jenkins/workspace/coreos-rhcos-aws-test/aws-us-east-1.json does not exist.

The root cause looks like this line:

+ rm -rf /home/jenkins/workspace/coreos-rhcos-aws-test

...so this should fix that.