Bug 1113245 - Remove cleanslate process list on Linux and Mac machines during reboots with halt.py; r=rail

This prevents the list from going stale on AWS hosts where puppet runs only during image
creation -- rather than at boot.
Bug 1103123 - Add runner task for restarting services on Linux machines; r=rail

In particular, xvfb must be restarted between jobs on test machines to prevent failures.