add 'force' verification in _volume_upload_image

_volume_upload_image API should only support to upload 'in-use' volume to
image when assign 'force' parameter as true value. Now
_volume_upload_image API does not do verification for 'force' parameter,
so we can upload 'in-use' volume to image when assign 'force' parameter
with string value 'false' or 'false111'. This patch add verification for
'force' parameter.

Change-Id: I9e1d0e517d0ff97c136903dd6e50a305e18651f3
Closes-Bug: #1265718
Merge "add 'force' verification in _volume_upload_image"