Save "META" field in Dockerfile into image content.

This will allow a user to save META 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 json 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.

One option is to call this flag Meta, another would be to call it Vendor.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
I think we could be a little bit more specific for API error messages: ideally validation error like this would list _verbatim_: the field, optionally their value and what was the failed. expectation.