Play nomad on all maps

[elexis]

The map "Unknown nomad" only supports about 12 different map types. This has two bad consequences:

1) Nomad experience is limited / repetitive. 2) Other maps are less frequently played.

Both can be solved by making the nomad-mode a gamesetup option.

Implementation: If this option is enabled, a trigger script could just spawn the starting entities and delete all other player entities (buildings, units).

Deleting pre-built cities in scenario maps might yield unexpected maps. This could be fixed by adding a boolean mapsetting which would deny nomad gamemode for that map.

The actual "unknown nomad" map would have to be deleted, as it seems to be identical to the map "Unknown" with nomad gamemode.

[elexis]

At least for random maps, this is going to become a low-hanging fruit following the commit that removes the (about 60 copies) of the starting base loop.