* Replace contextlib.nested() with contextlib.ExitStack
* Add contextlib2 dependency for Python 2.6 and 2.7 to get ExitStack
* TestLibvirtInspection: set libvirtError because it must be a subclass
of Exception on Python 3
Enable ceilometer.tests.compute.virt.libvirt.test_inspector on Python 3.
Change-Id: I82f4b911c1b3ede90805150630c222b1dd5f3474