cluster_discovery: simplify fetching bootstrap info

The original implementation of seeds-driven bootstrap had pieces of the
various validation required for bootstrap strewn about startup. Because
of this, I found it difficult to reason about what validations are done
when, and what work may be duplicated, while reading through the code.

This commit puts all validations up front so we determine whether we are
a founder immediately and use a cached value thereafter.

This is mostly addressing review comments on redpanda-data#6744.