This allows operators to delete large diskimage files after uploads
are complete, in order to save space.
A setting is also provided to keep certain formats, so that if
operators would like to delete large formats such as "raw" while
retaining a qcow2 copy (which, in an emergency, could be used to
inspect the image, or manually converted and uploaded for use),
that is possible.
Change-Id: I97ca3422044174f956d6c5c3c35c2dbba9b4cadf
fd454706ca