Code clean in node controller

With new wsme 0.5b5 we can use error status
code from exception attribute.

Change-Id: I7ef72854d4d54612e7023df24c69fcd56eb224ff
Use mock to do unit tests for pxe driver

OpenStack is moving to use mock instead of mox for unit tests
in order to be ready for python 3. This patch changes the
test cases for pxe driver from using mox to mock.

Change-Id: I96eb9de4cd7c93619c71840b4ce79d5b83f832e6
Closes-Bug: 1229398
Merge "Code clean in node controller"