Spring Boot supports the automatic configuration of an additional
HazelcastInstance if one already exists and an explicit property has been
set to use a different configuration for caching. So three cases are
supported really: no `HazelcastInstance` exists so we need to create one
anyway or an `HazelcastInstance` already exists; in that latter case, we
should either reuse it or create a new one.

Unfortunately, the conditions that checked those three use cases were
not ordered consistently and we could easily get in a situation where
both conditions were evaluated.

This commit makes sure that we  first check if an `HazelcastInstance`
exists and then (and only then) we create the missing `HazelcastInstance`
used for caching. The tests have also been improved to validate the
proper `HazelcastInstance` is used for caching.

Closes gh-5181