Making callback_url mandatory only for authorization_code and implicit

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@180755 a5903396-d722-0410-b921-86c7d4935375
fix for issue IDENTITY-1568

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@180756 a5903396-d722-0410-b921-86c7d4935375