phosphor-discover-system-state: set after multi-user.target

Set the phosphor-discover-system-state service to run after
multi-user.target to ensure the operation is successful.
Avoid setting the PowerRestoreDelay time still meet BMC not
being ready.
This is particularly important when facing network instability,
which can delay the activation of multi-user.target.

Test Log:
```
....
May 09 18:18:49 bmc phosphor-bmc-state-manager[903]: BMC_READY
May 09 18:18:49 bmc phosphor-bmc-state-manager[903]: Setting the BMCState field to xyz.openbmc_project.State.BMC.BMCState.Ready
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 1 If Enabled.
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 2 If Enabled.
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 3 If Enabled.
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 4 If Enabled.
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 5 If Enabled.
May 09 18:18:49 bmc phosphor-discover-system-state[1206]: The LastbootCause is a watchdog. Power policy execution is currently allowed
May 09 18:18:49 bmc systemd[1]: Started Reboot Host 6 If Enabled.
May 09 18:18:49 bmc phosphor-discover-system-state[1207]: The LastbootCause is a watchdog. Power policy execution is currently allowed
May 09 18:18:49 bmc phosphor-discover-system-state[1206]: One time not set, check user setting of power policy
May 09 18:18:49 bmc phosphor-discover-system-state[1206]: Host power is off, processing power policy xyz.openbmc_project.Control.Power.RestorePolicy.Policy.AlwaysOn
May 09 18:18:49 bmc phosphor-discover-system-state[1206]: power_policy=ALWAYS_POWER_ON, powering host on (0s delay)
May 09 18:18:49 bmc phosphor-discover-system-state[1207]: One time not set, check user setting of power policy
May 09 18:18:50 bmc phosphor-discover-system-state[1208]: The LastbootCause is a watchdog. Power policy execution is currently allowed
May 09 18:18:50 bmc systemd[1]: Started Reboot Host 7 If Enabled.
May 09 18:18:50 bmc phosphor-discover-system-state[1207]: Host power is off, processing power policy xyz.openbmc_project.Control.Power.RestorePolicy.Policy.AlwaysOn
May 09 18:18:50 bmc phosphor-discover-system-state[1207]: power_policy=ALWAYS_POWER_ON, powering host on (0s delay)
May 09 18:18:50 bmc phosphor-discover-system-state[1209]: The LastbootCause is a watchdog. Power policy execution is currently allowed
May 09 18:18:50 bmc phosphor-discover-system-state[1208]: One time not set, check user setting of power policy
May 09 18:18:50 bmc phosphor-host-state-manager[994]: Host state transition request of xyz.openbmc_project.State.Host.Transition.On
....

```

Change-Id: Ic97c331d41af6b3ca5673e711107d475362c72c3
Signed-off-by: Delphine CC Chiu <Delphine_CC_Chiu@wiwynn.com>
diff --git a/service_files/phosphor-discover-system-state@.service b/service_files/phosphor-discover-system-state@.service
index 2ac277e..5a7a64f 100644
--- a/service_files/phosphor-discover-system-state@.service
+++ b/service_files/phosphor-discover-system-state@.service
@@ -11,6 +11,7 @@
 After=phosphor-reset-chassis-on@%i.service
 Wants=xyz.openbmc_project.Settings.service
 After=xyz.openbmc_project.Settings.service
+After=multi-user.target
 ConditionPathExists=!/run/openbmc/chassis@%i-on
 
 [Service]