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/redfish/dmtf_tools/test_redfishtool_certificate.robot b/redfish/dmtf_tools/test_redfishtool_certificate.robot
index c3a53a9..7930a96 100644
--- a/redfish/dmtf_tools/test_redfishtool_certificate.robot
+++ b/redfish/dmtf_tools/test_redfishtool_certificate.robot
@@ -273,6 +273,7 @@
     ${cert_id}=  Redfishtool Install Certificate File On BMC
     ...  ${certificate_uri}  ${expected_status}  data=${file_data}
     Logging  Installed certificate id: ${cert_id}
+    Set Test Variable  ${cert_id}
 
     # Adding delay after certificate installation.
     Sleep  30s