readyness_check config variable placement

The following snippet is placed in `services.py`:

```
READYNESS_CHECKS = {
"es": es_healthcheck,
"postgresql": postgresql_healthcheck,
"mysql": mysql_healthcheck,
"redis": redis_healthcheck,
}
"""Readyness check functions module path, as string."""
```

However the rest of the configuration is placed in `config.py`. Should this be moved to `config.py`?
It might not trivial due to possible circular dependencies and importing a string is not trivial without depending on invenio-base.utils or duplicating the loading code. Moreover, there is no logic to allow the user to overwrite configuration with a provided `config.py` therefore having the configuration separated around files does not pose a problem.