GUI: Added Redfish login/logout step.

Changes:
    - In gui/gui_test/resource_management_menu/test_power_sub_menu.robot
    - Added Redfish login and logout step at the suite level.
    - Added wait until step to avoid success message popup for below test cases.

Tested:
    - Ran the above test case from the sandbox successfully

Change-Id: Iace52928831809b3fda6b25a37e22cc8fcf4858f
Signed-off-by: Nandish-Matti <nandish.matti@ibm.com>
diff --git a/gui/data/gui_variables.py b/gui/data/gui_variables.py
index f112ae7..dd92460 100644
--- a/gui/data/gui_variables.py
+++ b/gui/data/gui_variables.py
@@ -102,6 +102,7 @@
     )
     xpath_power_sub_menu = "//*[@data-test-id='nav-item-power']"
     xpath_power_heading = "//h1[contains(text(), 'Power')]"
+    xpath_success_message = "//*[contains(text(),'Success')]"
 
     # Profile settings
     xpath_default_UTC = "//*[@data-test-id='profileSettings-radio-defaultUTC']"