Add per policy container count to account stat output

When the account response includes 'X-Storage-Policy-X-Container-Count' the
command helpers will stick a helpful extra line in the output.

If the swift response doesn't include the information the behavior is
unchanged.

Change-Id: Iaaa55d207ebe6a9c16e52adb56eb858c57cfb26a
Merge "Add per policy container count to account stat output"