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/lib/resource.robot b/lib/resource.robot
index 6e54e6e..49ddc4c 100755
--- a/lib/resource.robot
+++ b/lib/resource.robot
@@ -35,6 +35,7 @@
 
 ${MANAGER_ID}              bmc
 ${CHASSIS_ID}              chassis
+${SYSTEM_ID}               system
 
 # MTLS_ENABLED indicates whether mTLS is enabled.
 ${MTLS_ENABLED}            False