Ticket #156: Remove useless object instantiations which were slowing loading.

-FSUtils.instantiate() was creating useless Server instances.
dict.setdefault() was creating Server instances even if it did not need
it because the entry already existed in the dictionnary.

-Configuration.iter_clients() does not need to parse node pattern.
This method was calling NodeSet() to parse node pattern, but, in this
was already done in Model. This is useless here.