Bucket config parser fails if $HOST is an IPv6 literal

The bucket config JSON uses $HOST placeholders in HTTP URLs and other contexts where it is followed by ":" and a port number. When an IPv6 literal occurs in these contexts, it must be enclosed in square brackets, but that is not currently happening.

This only affects environments where there is a single Couchbase server whose name is "localhost" or "127.0.0.1" (or "::1" I suppose, but I haven't tested) and the client bootstraps using a seed node with an IPv6 literal address (like "::1").