Add check_and_fence method and unit tests for NodeAgent class for nailgun agent

To run unit tests use:
rspec ./bin/agent_spec.rb --color --format documentation

Make sure you have:
ruby 1.9.3p429 / 1.8.7p371
rspec 2.14.7
---------------------
httpclient (2.3.4.1)
json (1.8.1)
logger (1.2.8)
ohai (6.20.0)
rethtool (0.0.3)

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

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