Instead of returning multiple services in simple-cuirass-services, rely on the
instantiate-missing-services procedure to instantiate postgresql and
postgresql-role-service-type when missing.

Turn simple-cuirass-services procedure into
simple-cuirass-configuration->specs, that takes a simple-cuirass-configuration
record and returns a Cuirass specification.

Suggested-by: Ludovic Courtès <ludo@gnu.org>

* gnu/services/cuirass.scm (%default-cuirass-config): Remove it.
(simple-cuirass-services): Rename it to ...
(simple-cuirass-configuration->specs): ... this procedure.
* gnu/tests/cuirass.scm (cuirass-services): Remove postgresql and
postgresql-role services that are automatically instantiated.
(simple-cuirass-service): New variable.
(%cuirass-simple-test): Adapt it to use simple-cuirass-configuration->specs
instead of simple-cuirass-services.
* doc/guix.texi (Simple Cuirass): Update it.