Bug 1103123 - Add post-flight runner task to linux slaves; r=rail

This task selectively triggers runner's halt task; which will reboot the machine.
This may be acheived via hostname blacklisting or discovery that a job has failed.

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extra : rebase_source : 09d6a4fd77c137f4e375155f599c51cccc880ef0
Bug 1103123 - pin test nodes to mphillips environment
Backout 1185781bb6c1 - post_flight task
Bug 961279 - Upgrade Mercurial clients to 3.2.1 r=dustin
Linux and OSX only.
Third times the charm
Merge from default

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branch : production