Add disks_active to configuration

Add disks_active to configuration

This flag tracks if the disks of an instace are supposed to be active.
That's the case when an instance is running or when its disks got
activated explicitly (and in a couple of other cases).
It will be used by watcher to re-activate disks after a node reboot.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>