[chassisd][thermalctld] Import os module, now needed for env var checking (sonic-net#113)

This was missed in sonic-net/sonic-platform-daemons#112

Author: jleveque

sonic-chassisd/scripts/chassisd

@@ -8,6 +8,7 @@
 """
 
 try:
+    import os
     import signal
     import sys
     import threading

sonic-thermalctld/scripts/thermalctld

@@ -6,6 +6,7 @@
 """
 
 try:
+    import os
     import signal
     import threading
     import time