ENV['MACHINES'] overrides [first second]

Integration tests need two virtual machines to complete. However, when
running a single integration test (via SPEC=...), it will save time to
not run the second virtual machine if it is not needed.

Change-Id: I0946c7e999186aebf5dc84661112518f63f45ce3
Signed-off-by: Loic Dachary <loic@dachary.org>