appRunner supports scaling an app

Signed-off-by: Matthew McNew <mmcnew@pivotal.io>
rename start_app_command_factory to app_runner_command_factory

Signed-off-by: Matthew McNew <mmcnew@pivotal.io>
rename AppRunnerCommandFactory to StartAppCommandFactory

Signed-off-by: Matthew McNew <mmcnew@pivotal.io>
Add scale diego app command

Signed-off-by: David Wadden <dwadden@pivotal.io>