Set system, chassis, and bmc as global variables

Changes:
  - Add SYSTEM_ID as a global variable and set to default value system
  - Replace redfish/v1/Systems/system to redfish/v1/Systems/SYSTEM_ID
  - Replace redfish/v1/Chassis/chassis to redfish/v1/Chassis/CHASSIS_ID
  - Replace redfish/v1/Managers/bmc to redfish/v1/Managers/MANAGER_ID

Tested:
  - Tested on sandbox with the changes with sample test run with
    sample test ID runs
    1. without SYSTEM_ID provided
    2. with -v SYSTEM_ID:system1
    3. with export SYSTEM_ID=""
    3. with export SYSTEM_ID="system2"

Change-Id: Ifc412b20038cef1eba632caaa6c300be328259bd
Signed-off-by: Wilson Suen <wilsons@supermicro.com>
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/ipmi/test_ipmi_sensor.robot b/ipmi/test_ipmi_sensor.robot
index 6a312c5..cd16d88 100644
--- a/ipmi/test_ipmi_sensor.robot
+++ b/ipmi/test_ipmi_sensor.robot
@@ -384,7 +384,7 @@
 
      # Redfish cpu components have "-" instead of "_" (e.g.: dcm0-cpu0).
      ${cpu_name}=  Replace String  ${sensor_name}  _  -
-     ${sensor_properties}=  Redfish.Get Properties  /redfish/v1/Systems/system/Processors/${cpu_name}
+     ${sensor_properties}=  Redfish.Get Properties  /redfish/v1/Systems/${SYSTEM_ID}/Processors/${cpu_name}
 
      #  Example of CPU state via Redfish