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