This commit separates the basic setup of Couchbase from Spring Data so
that a `Bucket` and `Cluster` bucket beans are exposed even if Spring
Data is not available.

A basic setup happens if `spring.couchbase.bootstrap-hosts` is specified,
configuring the `default` bucket with no authentication unless specified
otherwise.

If Spring Data is available, those beans are re-used by default to
configure the `CouchbaseTemplate` and other repository-related beans.

Closes gh-5347