[chassis][psud] Move the PSU parent information generation to the loop run function from the initialization function (#576)

Description
Move the PSU parent information generation to the loop run function from the initialization function

Motivation and Context
Fixes #575

How Has This Been Tested?
Tested on Cisco chassis, the PHYSICAL_ENTITY_INFO|PSU * can be re-inserted after thermalctld restart.
And monitored the stated db for memory for hours, works well:

Author: yejianquan

sonic-psud/scripts/psud

@@ -405,9 +405,6 @@ class DaemonPsud(daemon_base.DaemonBase):
         fvs = swsscommon.FieldValuePairs([(CHASSIS_INFO_PSU_NUM_FIELD, str(self.num_psus))])
         self.chassis_tbl.set(CHASSIS_INFO_KEY, fvs)
 
-        # Update predefined position_in_parent and parent_name for PSU
-        self._update_psu_entity_info()
-
     def __del__(self):
         # Delete all the information from DB and then exit
         for psu_index in range(1, self.num_psus + 1):
@@ -438,6 +435,13 @@ class DaemonPsud(daemon_base.DaemonBase):
             # We received a fatal signal
             return False
 
+        # Update predefined position_in_parent and parent_name for PSU
+        # it was in the __init__ function which means will be run only once
+        # But there's chance that the keys(PHYSICAL_ENTITY_INFO|*) got removed by other processes,
+        # like the exit function during the restart of thermalctld,
+        # hence move it to the iteration run, so the key will be filled again after removed.
+        self._update_psu_entity_info()
+
         self.update_psu_data()
         self._update_led_color()