Buildout cannot request destroy sub instances when it is removed from
instance parameters by the user. Ex: request a cluster of KVM, with
kvm1 and kvm2 instances, edit parameters and remove kvm2 in the list.
Buildout will just ignore that instance and it will be removed only when
the hosting subscription will be destroyed.

                 -- kvm1
root instance --|
                 -X- kvm2

So when processing a partition, we send to master the list of requested sub
instance(s). If the previous list was ['kvm1', 'kvm2'], then after remove kvm2
the list will be ['kvm1'] (for the root instance). Commit this list to the
master will unlink between root instance and kvm2 (predecessor categorie).

A new alarm will search for unlinked instances: Instance which has no parent,
and destroy them (also destroy the sub tree).