Allow for multiple image outputs from raw source

When uploading images to multiple clouds it is possible that the same
image will be needed in multiple formats to accomodate hypervisors
across clouds. Update disk-image-create's -t flag to take a list of
desired output image formats so that a single disk-image-create can
output all of the desired image formats.

Change-Id: If121b2342ae888855ba435aa3189f039e985b812
Merge "Allow for multiple image outputs from raw source"