target start order: MDT0 then MDTs then OSTs

[Dominique Martinet]

The start order is currently dynamic on first mount (a status is first issued to check writeconf flag), so MDTs are only started first if writeconf is set.

This causes problems in some DNE/HA setup where shine checks targets an even number of times and toggles the order as many time, leading to incorrect ordering - changing to fixed order should always work now (this used to be a bug that starting OSTs first work better) and is much simpler to handle.

For DNE, We also need the MDT0 to start first the first time, because secondary MDTs need to register to MDT0 as well.