Doc: Remove -r option for running a specific unit test
Doc: Remove -r option for running a specific unit test

Now that os-testr was replaced with stestr for running unit tests, -r
options is no longer necessary to pass a regex to the test runner.

Change-Id: I2d3738e3af47a4a01db3223941e07e68537b5a5c

openstack/ironic,openstack/ironic