Create ApplicationPid and remove SystemUtils

Create a new ApplicationPid class to remove the need for SystemUtils
and refactor existing calls.
Polish
Remove getEmbeddedServletContainers()

Remove the mutable getEmbeddedServletContainers() Map from
EmbeddedWebApplicationContext and instead use the `namespace` to
distinguish the management container.

The ServerPortInfoApplicationContextInitializer class replaces the
previous TestExecutionListener to exposes port properties (by
listening for EmbeddedServletContainerInitializedEvents).