license keyword can accept only some value

[gruel]

If the non-optional keyword access_right is set to open or embargoed an optional keyword can be added: license.
This keyword is by defaults set to cc-by for non-datasets and cc-zero for datasets but other license can be choose. They need to be in the list provided by the Open Definition License Service and only them or the upload of the metadata will return an error 400.

Need to implement a verification of the license for this two cases. The implementation seems more straightforward in the python part and probably more stable (don't need to update the list manually but use the one provided and maintain by the site).