LCG_91 Fixes, 1.0 branch (2018.01.15.)

Various fixes for using LCG_91, mostly in the 21.2 branch of atlas/athena. (Since the other 21.X branches will most likely not switch away from LCG_88 this year.)

I tried to make pip as usable as possible for the standalone releases. But unfortunately I failed at making it really usable. (Since on MacOS our privately built Python version doesn't have SSL capabilities.)

Still, these updates should fix a few issues.