There is no unit tests for "hypervisor" command. This patch
introudces a new class FakeHypervisor to fake one or more
hypervisors, and a base class TestHypervisor. Also adds
hypervisors mock to fake compute client.
And also, this patch adds unit tests for "hypervisor list"
command.
Change-Id: I18733eae1a8f4fff72e830d9a060fb8f0f58fbf5