[FLOC-1023] Allow an Application restart_policy to be configured in application.yml

Fixes https://clusterhq.atlassian.net/browse/FLOC-1023
In this particular case I could just test ` _parse_restart_policy` directly and add a test to establish that that function is called by the main config parser. 
I could also make the `ConfigurationError` accept an `application_name` argument and make it responsible for including the prefix "Application '{}' has a config error. "