htools: Use OpMigrateInstance with allow_failover option

Before hbal decided on the fly if an instance is migratable or not. As
we implemented failover fallback in commit d5cafd31 we can start to
use that.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>