Abstract Param upgrade from cluster.UpgradeConfig

A new UpgradeGroupedParams is used to upgrade all the profiles for one
parameter filling in the default values, or creating the whole dict
anew, should it be missing. This is used only for beparams, currently,
but will be used at least for nicparams and diskparams as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>