Cherry pick of "Merge branch 'AthDerivationExternalsFix-1.0-20170531' into '1.0'" into master

Fix the python setup in externals packages, 1.0 branch (2017.05.31.)

See merge request !56 (merged)

Jenkins Build SUCCESS

Results available at: Jenkins [AE-MERGE-REQUEST #78]

Since the 21.2 nightlies worked well with this update last night...

merged

mentioned in commit 642597f7