Upgrade be/hv params with default values

From time to time we're adding new be or hv parameters. With this patch
missing parameters get set to the default value when loading the cluster
object. This patch version also considers the case when hv/be params
don't exist at all, and fixes a broken unit test triggered in that
case.

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