Externals and flake8_atlas Update, master branch (2020.05.07.)

Updated all projects to atlasexternals-2.0.67.

The list of changes wrt. atlasexternals-2.0.65 are (atlasexternals@2.0.65...2.0.67):

• Copied POST_BUILD_CMD into atlas_install_generic(...), and made it a "list type" parameter;
  • Also updated the handling of ATLAS_PYTHON_CHECKER to deal correctly with list variables;
• Updated flake8_atlas not to fail when no python files are given to it, in case the user makes a mistake that would result in this;
• Made flake8_atlas process sub-directories correctly.

At the same time updated how AthGeneration and Athena would set up the usage of flake8_atlas during the build, by modifying the setup of ATLAS_FLAKE8 and ATLAS_PYTHON_CHECKER.

Also added a pre-configuration file to AthGeneration to make sure that when we build a package against an AthGeneration nightly/release, by default that build would use the same flake8_atlas settings as the underlying nightly/release.

This is to fix the issues @fwinkl advertised in

https://groups.cern.ch/group/hn-atlas-offlineSWDevelopment/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DofflineSWDevelopment%2FLists%2FArchive%2Fmaster%2Cr06%20broken&FolderCTID=0x01200200FBC5CE5673348640AB112EE6AB5B8297

, and which we discussed (mainly) in !32540 (comment 3432479).