Ubuntu 18.04 Fix(es), 1.0 branch (2019.01.21.)

The build of the latest version of Davix needs the Python interpreter. On a bare-bones machine that doesn't have python out of the box, this means that the build can not run until we've built Python ourselves.

It's actually very embarrassing that I completely forgot that I encountered this issue already in the past (https://github.com/root-project/root/pull/2829) when I updated to Davix-0.7.1.

Also, as you can see in https://github.com/cern-fts/davix/pull/38, once a new version of Davix will be put into the LCG release, our build configuration for it should simplify once more.

With this update AnalysisBaseExternals builds once again on my bare-bones Ubuntu 18.04 virtual machine.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST #690]

mentioned in commit 0b3fed96

merged

mentioned in commit 2bbae3e5

mentioned in merge request !426 (merged)