v1.3: Add option to run tests on 1 randomly-chosen device from the list. (#7)

* Add option to run tests on 1 randomly-chosen device from the list.

Due to hiccups in FTL availability, sometimes you may want to distribute
your test runs onto different FTL device types.

This option allows you to pass in a list of test_devices, but rather
than running the test on *all* devices in the list, you can run the
tests on one randomly-chosen device from the list instead.

* Fix typo

* Add random selection in the other part of the script.

* Remove extraneous device setting.

* Add readme note for new param.

* Format.

* Format.

* punctuation