Set network_scope group as parent of network groups

All groups linked to a network are automatically added to the group
linked to their network scope if it is defined.

Note that it's not allowed to defined manual parent - child relationship
between network groups or network scope groups (only the one based on
scope/network is added automatically).

WARNING! Do not use children.append() as it bypasses the setter. It will
append the element to the object returned by the getter (as it's not the
self._children list, it won't work anyway).

JIRA INFRA-2639 #action In Progress