default_ipv6_prefix takes any value, and produces non-compliant addresses

[cynthia]

default_ipv6_prefix will take any value - and happily generates non-compliant IPv6 addresses.

A common mistake is for users to put in prefixes like:

"default_ipv6_prefix": "aabb:ccdd:1122:3344"

When the expectation is

"default_ipv6_prefix": "aabb:ccdd:1122:3344:"

emphasis on the last colon. There should be some handling for problematic input like this.

(Let's set aside the minor quirk that the address that comes out of the example is probably non-compliant too.)