Load interval from thermal_policy.json (sonic-net#178)

Description
Update ThermalManagerBase to load an interval field from thermal_policy.json if it is available and provide a getter for this interval.

Motivation and Context
This interval can be used by thermalctld to run policies at an interval specified in thermal_policy.json, rather than a fixed constant (currently 60 seconds).

How Has This Been Tested?
Verified that thermalctld runs without exiting.
Also ran test_thermalctld.py

sonic_platform_base/sonic_thermal_control/thermal_manager_base.py

@@ -14,6 +14,7 @@ class ThermalManagerBase(object):
     JSON_FIELD_THERMAL_ALGORITHM = "thermal_control_algorithm"
     JSON_FIELD_FAN_SPEED_WHEN_SUSPEND = "fan_speed_when_suspend"
     JSON_FIELD_RUN_AT_BOOT_UP = "run_at_boot_up"
+    JSON_FIELD_INTERVAL = "interval"
 
     # Dictionary of ThermalPolicy objects.
     _policy_dict = {}
@@ -25,6 +26,8 @@ class ThermalManagerBase(object):
 
     _run_thermal_algorithm_at_boot_up = None
 
+    _interval = 60
+
     _running = True
 
     @classmethod
@@ -118,7 +121,8 @@ def load(cls, policy_file_name):
                 }
               ]
             }
-          ]
+          ],
+          "interval": "30",
         }
         :param policy_file_name: Path of JSON policy file.
         :return:
@@ -147,6 +151,10 @@ def load(cls, policy_file_name):
                     cls._fan_speed_when_suspend = \
                         int(json_thermal_algorithm_config[cls.JSON_FIELD_FAN_SPEED_WHEN_SUSPEND])
 
+            if cls.JSON_FIELD_INTERVAL in json_obj:
+               cls._interval = int(json_obj[cls.JSON_FIELD_INTERVAL])
+
+
     @classmethod
     def _load_policy(cls, json_policy):
         """
@@ -215,3 +223,10 @@ def init_thermal_algorithm(cls, chassis):
                     for psu in chassis.get_all_psus():
                         for fan in psu.get_all_fans():
                             fan.set_speed(cls._fan_speed_when_suspend)
+
+    @classmethod
+    def get_interval(cls):
+        """
+        Get the wait interval for executing thermal policies
+        """
+        return cls._interval