[atlas] modifications to Atlas front-end servers

dear Pier, Bjørn and all, We are in the process of migrating the atlas front-end machinery from apache to nginx. Despite our extensive preparations, this change has not been completely transparent, as you have noticed, and has caused some problems with the certificate chains. The problems with the CERTs have now been fixed. Apologies for this inconvenience to everyone. Please let us know if you would spot any other anomalies in the behaviour of the Atlas service. Kind regards, Romeo Zwart