Make application PID file location configurable from Environment propert...

Make application PID file location configurable

The ApplicationPidListener class can now be configured to search the Environment for the occurrence of the property 'spring.application.pid-file' in order to configure the location of the PID file from the central applications configuration file. Sticks to defaults if the property is not found.

Fixes gh-1579
Rather than using a boolean, I think @philwebb's suggestion of taking the event class that should be listened for would be a more flexible approach. Depending on the type of the event, the environment can then be used it it's available. If the environment's not available or it doesn't contain the property to configure the pid file location, the default location could be used.