Allow specification of UserData Name/Value pairs in image json content

Save "UserData" field in Dockerfile into image content.

This will allow a user to save user data into an image, which
can later be retrieved using:

docker inspect IMAGEID

I have copied this from the "Comment" handling in docker images.

We want to be able to add Name/Value data to an image to describe the image,
and then be able to use other tools to look at this data, to be able to do
security checks based on this data.

We are thinking about adding version names,
Perhaps listing the content of the dockerfile.
Descriptions of where the code came from etc.

This USERDATA field should also be allowed to be specified in the
docker import --change USERDATA:Name=Value
docker commit --change USERDATA:Name=Value

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)