Allow to determine IPv6 routing strategy
Summary:
The method network_utils.can_directly_be_discovered can be invoked
to determine if we can directly reach the gateway (HE for example)
or if we need an intermediate route to that gateway (OVH for example).
Ref T1853
Test Plan: Unit tests provided for new Python functions
Reviewers: dereckson
Reviewed By: dereckson
Maniphest Tasks: T1853
Differential Revision: https://devcentral.nasqueron.org/D3077