Remove redfish login and logout from lib code

With this change, the test script caller is expected to have the
valid session before invoking the changed keywords in library.

Changes:
      - Remove login/logout from lib/bmc_redfish_utils.robot
        for power and reset operation.
      - Update impacted test suites.

Tested: To identify the impact, ran the existing test bucket and
        the result is looking good.

Change-Id: I6df69d6fcc4c38470fdb2ec3b31fb28beb726d70
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/redfish/systems/test_systems_inventory.robot b/redfish/systems/test_systems_inventory.robot
index dbbc833..606cf09 100755
--- a/redfish/systems/test_systems_inventory.robot
+++ b/redfish/systems/test_systems_inventory.robot
@@ -159,7 +159,6 @@
     [Tags]  Check_GPU_States_When_Power_Off
 
     Redfish Power Off
-    Redfish.Login
     GPU State Check
 
 
@@ -245,8 +244,8 @@
 Suite Setup Execution
     [Documentation]  Do test case setup tasks.
 
-    Redfish Power On  stack_mode=skip
     Redfish.Login
+    Redfish Power On  stack_mode=skip
 
     ${cpu_info}  ${gpu_info}  ${power_supplies}=  Get Inventory URIs