Add check and fence method for NodeAgent class for nailgun agent

Implements: https://blueprints.launchpad.net/fuel/+spec/fencing-in-nailgun-agent

Change-Id: Ic7d9d38a65514a89aa1581109757fb1ed8a6702c
Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>