Bug 9045 - Update netconf readiness suite

Bug 9045 - Update netconf readiness suite

netconf-connector readiness test suite checks if system is ready to
mount netconf devices. This is done by mounting CSS netconf endpoint and
trying to access
config:modules/module/sal-restconf-service:json-restconf-service-impl
/json-restconf-service-impl
resource located on mounted CSS netconf server.

However, this resource was deprecated and removed in Nitrogen release.
Check ietf-netconf-monitoring:netconf-state resource instead. Netconf
monitoring is supported on CSS netconf endpoint, so this resource is
present.

Change-Id: I8525cbcf8803046f512de306aacd84f536793447
Signed-off-by: Jakub Morvay <jmorvay@cisco.com>