Add capability to have split configuration files in a nuxeo.conf.d folder

The nuxeo.conf file is getting pretty big.

A solution would be to just keep the generic options there, and have a nuxeo.conf.d folder with additional configuration files:

nuxeo.conf
nuxeo.conf.d/database.conf
nuxeo.conf.d/elasticsearch.conf
...

This kind of split files configuration would also be nice for automation as instead of modifying the main nuxeo.conf you could just contribute a file in nuxeo.conf.d with your configuration changes.

However, several things need to be determined for this kind of configuration:

• what is the priority order? (if I have a setting in nuxeo.conf and the same one in nuxeo.conf.d/something.conf, which one has precedence?)
• when a configuration modification is made through the admin center or nuxeoctl, which files should be modified? Should we track where a setting comes from?
• contributing a file with (for instance) db settings is nice, but there is still the matter of the template used.
  Ideally, we would also specify that in the same file, but that it not possible with the current "nuxeo.templates" setting.
  We may want to add something like "nuxeo.template.add=..." that would just add a template to the list for a regular template, but also be "database template aware" and in this case replace the db template instead of adding to the list.
  Or possibly split db templates in another variable.