readyness_check config variable placement

[ppanero]

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.