ATLAS LCG_95rc1 Fix, master branch (2019.02.07.)

Made the logic in GaudiProjectConfig.cmake compatible with the GAUDI_ATLAS=TRUE mode.

This required delaying the finding of Python and Boost until the call to gaudi_project(...), to let the base ATLAS project set everything up for finding these things correctly.

At the same time updated the default ATLAS project name to "AthenaExternals". Since that has been our offline software's external project for some time now.

This is my proposal for the fix instead of !846 (closed). 😛 @wlampl, I was able to build the HEAD of Gaudi against this branch of atlasexternals:

https://gitlab.cern.ch/akraszna/atlasexternals/tree/LCG_95rc1Testing-master-20190207

(I had to add FindVDT.cmake to our code.)