Ensure that spring.data.rest.* configuration takes effect

Previously, RepositoryRestMvcBootConfiguration was not annotated with
@Configuration. This meant that it was processed in lite mode.
Crucially, in lite mode, there’s no proxying so each call to the
config() @Bean method from within other @Bean methods resulted in the
creation of a new RepositoryRestConfiguration instance. Furthermore, as
each of these instances wasn’t a Spring bean the configuration
properties were not applied.

This commit updates RepositoryRestMvcBootConfiguration to annotate it
with @Configuration so that it’s no longer processed in lite mode. It
also updates the unit tests and the Spring Data REST sample to verify
that the baseUri can be configured using application.properties.

Fixes gh-1675