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>
reminder to run a single integration test

Running a single integration test in a development environment is
convenient but the sequence to do so is not trivial to remember. It is
added to the end of the ceph_key_spec.rb test file, using an emacs
syntax so it can be run with M-x compile.

* It only creates a single virtual machine instead of two
* It only tests on cuttlefish, assuming that it is likely to work on
  newer versions as well
* The virtual machine is not destroyed at the end of the test to help
  with debugging

Change-Id: I2e18b8453fcba905cb99292bfd5e950a2900e185
Signed-off-by: Loic Dachary <loic@dachary.org>