WaitProcessOutputListener needs to be provided by the AppEngineDevServer implementation

[patflynn]

related to #330

currently CloudSdk#637 hardcodes the value that triggers server startup complete event.

[patflynn]

@loosebazooka we need this fix in for the IDE integrations to work with Ludo's PR.

[loosebazooka]

Is this a stdout vs stderr thing? The string we are looking for is correct.

[chanseokoh]

@loosebazooka Oh, so devappserver1 and 2 both use the same log message when starting up?

".*(Dev App Server is now running|INFO:oejs\\.Server:main: Started).*",

[loosebazooka]

dev appserver 1 is the first part and dev appserver 2 is the second.

[chanseokoh]

Got it. I thought this issue is talking about refactoring the code so that the expected log messages are provided by server implementations rather than hard-coded in CloudSdk, but we may be okay as long as it's working.