Allow instance disc activation with offline secondaries

Currently, this is not allowed, so one can't run a replace-disks; this
breaks any non-invasive method of recovering the redundancy of the
instance if its disks are already stopped (but it still works if the
disks on the primary are active). So let's fix this inconsistency.

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