Render countries capital based on their admin_centre nodes

[naoliv]

As per #2314 (comment) and #2314 (comment), it would be good if we could have the countries capitals rendered based exclusively on their admin_centre nodes, instead using nodes with capital=yes + admin_level=2

ie, querying for the admin_centre node of admin_level=2 relations

An example in overpass for South America:
http://overpass-turbo.eu/s/ixw

This should solve old discussions like https://wiki.openstreetmap.org/wiki/Talk:Proposed_features/capital#Use_the_relation_boundary and also avoid adding an unnecessary admin_level tags in the nodes (which seems to be only a workaround for renderers)

An example for this: Brasília is the admin_centre of 3 relations: an admin_level=2, admin_level=4 and an admin_level=8

By having admin_level=2 in its node, just to have it rendered as a capital, has one issue: people looking at the node will think that Brasília is only part of an admin_level=2 relation/area (which is an incomplete information). This is basically the discussion here

The Humanitarian style also has the same issue hotosm/HDM-CartoCSS#281

[pnorman]

We do not have this information in the database. osm2pgsql does not have membership information and nodes as relation members do not make it into geometries.