slap/standalone: Use IPv6 range when available

StandaloneSlapos.format can now take an ipv6 range (CIDR notation <ipv6>/N) instead of an ipv6 address (without /).

The logic is then something like:

if ipv6 is a range with prefix length N < 112:
  => give a unique ipv6 to each partition and a unique /N+16 range
if ipv6 is a range with prefix length >= 112
  => give a unique ipv6 to each partition
if ipv6 is not a range:
  => give the same ipv6 to each partition