minor issue: Some maps spawn without the usual 3/4 mexes near the ACU

[baurse]

As can be seen in this replay: https://replay.faforever.com/15259120 sometimes there is no cluster of 3/4 mexes near the ACU spawn location, resulting in the lack of a 'main base'.

While this can lead to interesting matches, it should probably be a parameter one can explicitly turn up or down.