Fix zuul systemd files

Our current systemd unit files do not work with type=forking, it does
work with type=simple. This also means we can drop the PIDFile, as it
is only used by forking.

Change-Id: Ia9a2e54b91e9b2430c612b2edfa07077f7248539
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

etc/zuul-executor.service

@@ -3,13 +3,12 @@ Description=Zuul Executor Service
 After=syslog.target network.target
 
 [Service]
-Type=forking
+Type=simple
 Group=zuul
 User=zuul
 RuntimeDirectory=zuul
 ExecStart=/usr/local/bin/zuul-executor
 ExecStop=/usr/local/bin/zuul-executor stop
-PIDFile=/var/run/zuul/executor.pid
 
 [Install]
 WantedBy=multi-user.target

etc/zuul-scheduler.service

@@ -3,13 +3,12 @@ Description=Zuul Scheduler Service
 After=syslog.target network.target
 
 [Service]
-Type=forking
+Type=simple
 Group=zuul
 User=zuul
 RuntimeDirectory=zuul
 ExecStart=/usr/local/bin/zuul-scheduler
 ExecStop=/usr/local/bin/zuul-scheduler stop
-PIDFile=/var/run/zuul/scheduler.pid
 
 [Install]
 WantedBy=multi-user.target

etc/zuul-web.service

@@ -3,13 +3,12 @@ Description=Zuul Web Service
 After=syslog.target network.target
 
 [Service]
-Type=forking
+Type=simple
 Group=zuul
 User=zuul
 RuntimeDirectory=zuul
 ExecStart=/usr/local/bin/zuul-web
 ExecStop=/usr/local/bin/zuul-web stop
-PIDFile=/var/run/zuul/web.pid
 
 [Install]
 WantedBy=multi-user.target