Restore "check.repositories.running" property

From a June 28, 2016 e-mail from Woonsan Ko on the hippo-community mailing list (see https://www.onehippo.org/library/about/cms-forums.html?place=msg%2Fhippo-community%2FRU56ypEcbuc%2FSGYZDFWBAwAJ):

We used to have an option (check.repositories.running = true in hst-config.properties) to have HST Container initialization wait for the underlying repository really responding, but unfortunately the option was disabled since Hippo 10.2. [1] We forgot its goodness at the moment.

Please feel free to create an HSTTWO issue for an improvement.

[1] https://issues.onehippo.com/browse/HSTTWO-3395

The intention is to provide a notification mechanism for when the Jackrabbit repository is up and running. This would be helpful for things like initializing a REST client with the configuration values stored in the repository (which is something the "Integrate with an External REST API" example currently does not do).