Simplify list container output

This commit reduces the indentation of blob names and makes parsing
empty container easier for scripts.  The previous format:

containerName1                           <empty>

containerName2                           blobName1
                                         blobName2
                                         blobName3

The new format:

containerName1:

containerName2:

    blobName1
    blobName2
    blobName3