Handle Git connection issues when fetching data

[sashadidukh]

When JGitEnvironmentRepository fetching data (see fetch method)
and Git is unavailable at this moment (for example we got UnknownHostException), we just log WARN message and return null FetchResult.

I propose to send some ApplicationEvent in this case for adding the possibility to handle this situation.

[ryanjbaxter]

What would the application do in the case of this event?

[sashadidukh]

For example, we have Git repository with some configuration for Config Client application.
We change the configuration in the Git repository, and the Git notify changes through a webhook to the Config Server application.
Config Server application will send a RefreshRemoteApplicationEvent targeted at the applications it thinks might have changed.
Our Config Client application go to the Config Server application to obtain new configuration and at this moment Git became unavailable and Config server application return previously cached configuration.

In this case, some ApplicationEvent can help us to create some scheduler with checks to Git availability and notifying Config Client application that new configuration can be obtained now.
Also, it will be useful for the metric purpose.

[ryanjbaxter]

Sounds like this PR might address some of your concerns #677

[sashadidukh]

Thanks, but if we want to get configuration in JSON format and use labelledJsonProperties EnvironmentController method, we can't get cached from Environment as done in #677.
Looks like it's better to add this cached flag to the Response Header.

[ryanjbaxter]

I think its best to comment on the PR and see if we can make it work for your use case.