Log a message when a lifecycle event finished

[mpdude]

I am using log collection/processing software to monitor the CodeDeploy agent logs and spot errors during the deployment process.

Sometimes it is hard to tell if a particular Lifecycle Event has been processed successfully: When looking for strings that indicate failure, you cannot tell for sure if they did not occur or if the event is still running some time-consuming task and thus you need to keep on watching.

So, add an extra log message indicating when a lifecycle event has been processed.

Fixes #270.