Fixing restconf_client.py script

Fixing restconf_client.py script

The restconf_client.py script uses BlueprintWebClientService to send
REST requests to ODL. When the methods of that interface have the type
of the returns changed from String to WebClientResponse, the script
stopped working.

This fix changes the way the response is manipulated, to act upon a
WebClientResponse and not on a String

Change-Id: I3876c5b27d65058f0b5b77da17f9225fc04779c6
Issue-ID: CCSDK-1384
Signed-off-by: ottero <rodrigo.ottero@est.tech>