Certificate: Hardcoded keybit length was removed and an keyword was introduced to have keybit length in variables section

Changes:
    - ${keybit_length} keyword was introduced in variables section.
    - keybit length(2048) declared inside test case was renamed to ${keybit_length}.
    - Introduced ${keybit_length} variable in lib/certificate_utils.robot file.
    - Renamed hardcoded keybit length in "Generate Certificate File Via Openssl" keyword.

Tested:
     - Ran Following Scripts Successfully
        - redfish/dmtf_tools/test_redfishtool_certificate.robot
        - redfish/managers/test_certificate.robot

Change-Id: I9f6490acb468a61d4cd35b5082b770016f293005
Signed-off-by: ganesanb <ganesanb@ami.com>
diff --git a/lib/certificate_utils.robot b/lib/certificate_utils.robot
index 9930aac..961178c 100755
--- a/lib/certificate_utils.robot
+++ b/lib/certificate_utils.robot
@@ -9,6 +9,7 @@
 
 # Default wait sync time for certificate install and restart services.
 ${wait_time}    30
+${keybit_length}  2048
 
 *** Keywords ***
 
@@ -85,7 +86,7 @@
 
     Check If Openssl Tool Exist
 
-    ${openssl_cmd}=  Catenate  openssl req -x509 -sha256 -newkey rsa:2048
+    ${openssl_cmd}=  Catenate  openssl req -x509 -sha256 -newkey rsa:${keybit_length}
     ...  ${SPACE}-nodes -days ${time}
     ...  ${SPACE}-keyout ${cert_dir_name}/cert.pem -out ${cert_dir_name}/cert.pem
     ...  ${SPACE}-subj "/O=XYZ Corporation /CN=www.xyz.com"