[Merge] ~3v1n0/ubuntu/+source/gnome-session:ubuntu/master into ~ubuntu-desktop/ubuntu/+source/gnome-session:ubuntu/master

[Didier Roche]

Review: Disapprove

Let's just use getenv which is better for this IMHO. I'm adding also a missing import sys.
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https://code.launchpad.net/~3v1n0/ubuntu/+source/gnome-session/+git/gnome-session/+merge/354394
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