Add support for <candidate> validation

Positive Response:

If the device was able to satisfy the request,
an <rpc-reply> is sent that contains an <ok> element.

Negative Response:

An <rpc-error> element is included in the <rpc-reply>
if the request cannot be completed for any reason.
A validate operation can fail for any of the following reasons:
+  Syntax errors
+  Missing parameters
+  References to undefined configuration data

<rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <validate>
         <source>
           <candidate/>
         </source>
       </validate>
     </rpc>

     <rpc-reply message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <ok/>
     </rpc-reply>