[Feature] Alternative listing of services

[fmigneault]

As of latest versions, GET /services returns services nested under their respective service-type, which are then again provided under each element's service_type field.
There is therefore redundancy of information, but even more, this format makes it hard to parse since one always needs to loop service-type keys to then exact sub-services keys, to finally obtain a flat list of "service" objects.

A query parameter list=true|false should be added to render the same information as flat list of service objects.
Original format must still be returned for backward compatibility (an avoid massive rewrites), but the new flag could help skipping a few steps by requesting user that wants it as such.