Modify VTN Test files and VTNMgr Keyword file to use RESTCONF instead of the REST API

Modify VTN Test files and VTNMgr Keyword file to use RESTCONF instead of the REST API

    * Renamed the below files name to functionality specific
        010__vtn_manager.robot -> 010__vtn_vbridge_portmapping_test.robot (VTN_Mgr_OF10 and VTN_Mgr_OF13)
        010__restconfig_vtn_manager.robot -> ../VTN_Mgr_OF13/010__flowcondition_test.robot
    * The above test files is modified to work for RESTCONF specs and remaining robot files tests are disabled with Tags exclude
    * Modify VtnMaKeywords.robot library file according to RESTCONF specs
    * Tags exclude in robot files will be removed once each test file is modified and tested for RESTCONF specs.

Change-Id: Idcfea115300f0651a4a58d9f5a6eb117a78b1182
Signed-off-by: Karthik.S <siva-karthik@hcl.com>