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/redfish/service_root/test_service_root.robot b/redfish/service_root/test_service_root.robot
index 9e4c2ba..9f83e2e 100644
--- a/redfish/service_root/test_service_root.robot
+++ b/redfish/service_root/test_service_root.robot
@@ -49,7 +49,7 @@
     # Expect status      Resource URL Path
     ${HTTP_OK}           /redfish/v1/SessionService
     ${HTTP_OK}           /redfish/v1/AccountService
-    ${HTTP_OK}           /redfish/v1/Systems/system
+    ${HTTP_OK}           /redfish/v1/Systems/${SYSTEM_ID}
     ${HTTP_OK}           /redfish/v1/Chassis/${CHASSIS_ID}
     ${HTTP_OK}           /redfish/v1/Managers/${MANAGER_ID}
     ${HTTP_OK}           /redfish/v1/UpdateService