Use prefixlen 56 for OVH IPv6

Summary:
OVH assigns a router address OUTSIDE the routed /64 prefix.

Ref T1853.

Test Plan: Set several times in live. Stabilized enough to commit.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1853

Differential Revision: https://devcentral.nasqueron.org/D3519