Delete swift segments when image_size_cap exceeded

When an image which is being written to swift in segments exceeds the
'image_size_cap' limit we should ensure that the now stale swift segments
are deleted.

Fixes bug 1154576.

Change-Id: I08be04e63f6d55acc6b0350810350a4ccadab7be
Merge "Delete swift segments when image_size_cap exceeded"
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp