[Tile-serving] [openstreetmap/mod_tile] Added build & test on CentOS 7 (#247)

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

* There are more things in the PR, that would definitively good improvements. Maybe it would be worth to separate the two!?
* I created an `centos7` branch. Do you think this could be a good place to store for the building logic as reference for others? And I would like to have an option to check by manually triggering the build for `centos`. This branch can be regularly rebased on `master`.

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