Make sla-len config optional

[David Beveridge]

When the sla-len is not provided in the config file it is assumed the ISP will allocate a /48 and wide-dhcpv6 will assume the site level aggregator is always 16 bits.
If the ISP only allocates a /56 and the sla-len is not set to 8, then an error is generated.

This patch prevents the error from being displayed and instead calculates the sla-len.
so for /48, 64-48 is 16
and /56 64-56 = 8
/60 is 4 etc.

This means that the configuration file is works without the sla-len being specified even when the Prefix Delegated is not and exact /48.

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "wide-dhcpv6-0008-Make-sla-len-config-optional.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]