Simplify file reading and catch exception for type converting.

slapos/grid/slapgrid.py

@@ -575,14 +575,15 @@ class Slapgrid(object):
         self.instance_root, computer_partition_id)
       timestamp_path = os.path.join(instance_path, '.timestamp')
       if os.path.exists(timestamp_path):
-        descriptor = open(timestamp_path)
-        old_timestamp = int(descriptor.read())
-        descriptor.close()
+        old_timestamp = open(timestamp_path).read()
         parameter_dict = computer_partition.getInstanceParameterDict()
         if 'timestamp' in parameter_dict:
-          timestamp = int(parameter_dict['timestamp'])
-          if timestamp <= old_timestamp:
+          timestamp = parameter_dict['timestamp']
+          try:
+            if int(timestamp) <= int(old_timestamp):
               continue
+          except ValueError:
+            pass
       try:
         software_url = computer_partition.getSoftwareRelease().getURI()
       except NotFoundError: