Reland "Change cleanup phase to only restart the UI instead of rebooting the machine."

This reverts commit 9479fcddc9a8578c91c844cba13243f0a4e09cbd.

Now that we've converted platform_Shutdown to a server-side test, we can safetly commit
this change.

Previously reviewed in: I07525959ea3cb76b1c0ec413b9a968660e92f832

BUG=chromium-os:34489
TEST=Ran the cleanup phase directly with autoserv with argument
server/control_segments/cleanup. I also started up a local autotest
server and ran a Job which ran through client cleanup phases.

Change-Id: I1892a2e7a9fd012461342771fe554e8f5ea59532
Reviewed-on: https://gerrit.chromium.org/gerrit/36922
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Ready: Chris Sosa <sosa@chromium.org>
4 files changed