Running ./unity --replace ... terminates the session and starts a new one on vivid

[ Description ]

Due to an error when merging with Debian, ubuntu's gnome-session file is
inadvertantly starting compiz when it should instead be managed by
upstart. This means that we don't, for example, have unity restarted by
upstart if it crashes and (as the reporter of this bug notes) unity
--replace breaks due to gnome-session tearing the session down if one of
its required components goes away and comes back.

[ Fix ]

Remove "compiz;" from ubuntu.session so that the unity7.conf shipped by
unity is used.

[ QA ]

Verify that you see the following before upgrading

laney@raleigh> status unity7
unity7 stop/waiting
laney@raleigh> pstree -s -p $(pidof compiz)
systemd(1)───lightdm(1625)───lightdm(10651)───upstart(10864)───gnome-session(11094)───compiz(11374)─┬─{dconf worker}(11382)
                                                                                                    ├─{gdbus}(11383)
                                                                                                    └─{gmain}(11376)

i.e. compiz is a child of gnome-session, and the unity7 job isn't
running.

and this after

laney@raleigh> status unity7
unity7 start/running, process 8945
laney@raleigh> pstree -s -p $(pidof compiz)
systemd(1)───lightdm(1625)───lightdm(8487)───upstart(8715)───compiz(8945)─┬─{dconf worker}(8972)
                                                                          ├─{gdbus}(8976)
                                                                          └─{gmain}(8950)

compiz is a direct child of upstart and the unity7 job is running.

Additionally, verify that unity --replace kicks you back to the display
manager before and unity is replaced in-session after.

[ Regression potential ]

unity's job is now used instead of upstart being started by
gnome-session. This was the case since we started having upstart in the
session, but make sure that login and logout still work properly.

[ Original description ]

Running ./unity --replace <params> on vivid (systemd) exits the session
and the login screen of lightdm appears. After log in, a new session
starts.

Steps:
1- build unity
2- run ./unity --replace with the env. variables and parameters you used
   on utopic
3- see the lightdm login screen and login

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: ubuntu-session 3.14.0-2ubuntu4
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Uname: Linux 3.19.0-15-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Apr 29 13:15:16 2015
InstallationDate: Installed on 2014-07-04 (299 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: gnome-session
UpgradeStatus: No upgrade log present (probably fresh install)