For stand-alone operation, use 'pm2' as the execution context for 'appium'

Selenium Grid node servers implemented in Java that extend RemoteWebDriver are launched via GridLauncherV3, automatically spawning as stand-alone processes. By contrast, Appium runs as a child process of the execution context that launches it. This doesn't present any particular challenge when running a collection of tests that use Selenium Foundation to manage a Grid instance that exists only while the tests are running.

For execution as a stand-alone process, I added the ability to use the PM2 process manager as the execution context for the Appium node server. This feature must be explicitly activated with the APPIUM_WITH_PM2 configuration settings.