Fix for redfishtool certificate, snmp and session management

Changes:
  - Set cert_id as test variable
  - Remove colon in snmp destination when the snmp port is empty
  - Fix redfish_delete_sessions assign location
  - Set REDFISH_DELETE_SESSIONS to 0 in suite setup
  - Set REDFISH_DELETE_SESSIONS back to 1 in suite teardown

Tested:
  - redfish/service_root/test_sessions_management.robot
  - redfish/dmtf_tools/test_redfishtool_certificate.robot
  - redfish/events/test_bmc_snmp_trap.robot

Change-Id: Ibb16d52b5874484f8cf169456e2feb4ba7593e71
Signed-off-by: Wilson Suen <wilsons@supermicro.com>
diff --git a/lib/obmc_boot_test.py b/lib/obmc_boot_test.py
index 8e0c17e..5b3d7de 100755
--- a/lib/obmc_boot_test.py
+++ b/lib/obmc_boot_test.py
@@ -1077,6 +1077,11 @@
     global boot_success
     global boot_end_time
 
+    # The flag can be enabled or disabled on the go
+    redfish_delete_sessions = int(
+        BuiltIn().get_variable_value("${REDFISH_DELETE_SESSIONS}", default=1)
+    )
+
     gp.qprintn()
 
     next_boot = select_boot()