make source and target element optional in callout mediator

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@180227 a5903396-d722-0410-b921-86c7d4935375
Fixing https://wso2.org/jira/browse/WSAS-1431

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@180228 a5903396-d722-0410-b921-86c7d4935375
removing obsolete variable

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@180229 a5903396-d722-0410-b921-86c7d4935375