Create baymodel copy from an existing one

Sometimes user want a new baymodel which only have one or a few different properties with existing one. For example, user just want a new baymodel with a new image. So it's convenient to create new baymodel with copying properties from an existing one and overriding specified properties by user.

Changes:
add "--copy-from" argument into magnumclient
user execute "magnum baymodel-create --copy-from <baymodel_ident> --image-id <new-id> --name <new_name> --*** <new_value>"
magnum-api copy properties from baymodel<baymodel_ident> and override the specified properties, then create new baymodel.