do not generate config files when doing checks and other read-only operations

[WolfgangFahl]

mwcluster --check        
checking docker access for mediawiki versions ['1.27.7', '1.31.16', '1.35.10', '1.37.6', '1.38.6', '1.39.3']
... .pymediawikidocker/mw-127/Dockerfile already exists!
...