tap_ipv6 option let you chose if you want IPv6 on the taps or not. Some
customers may decide to have only IPv4 inside their VMs.

We put only 1 address on the tap itself (e.g.
2001:67c:1254:e:89:69b0:ffff:ffff/128 instead of
2001:67c:1254:e:89:69b0:ffff:ffff/96) otherwise, when we assign the
address 2001:67c:1254:e:89:69b0::/96 inside the VM, the address has
"dadfailed" status (dad = "Duplicate Address Detection"). The problem
appears only with address "0" (ending in ::) but not with others
addresses. We had 2 solutions:
 * put only one address on the tap (the solution we choose)
 * forbid address "0" inside the VM

The chosen solution has the advantage that the host machine won't
answer the ping on behalf of the VM since the tap has only 1 address
(its own).

/cc @alain.takoudjou

/reviewed-on nexedi/slapos.core!86