Require container & disk formats on image create
    
    Fixes lp 933702
    
    For images created via the glance CLI, the container and disk formats
    were previously defaulted if not explicitly set. However if created via
    the python or REST APIs, these attributes were not defaulted if unset.
    
    There is no real sensible default for these formats, so now an image
    create fails with 400 "Bad Request" if the format metadata are missing.
    
    Also we ensure unset image metadata are not reported in x-image-meta-*
    headers in order to disambiguate None and empty string values.
    
    Change-Id: I8189383f5f9adf42a8cdac7f8dc7e9327baf46da