Documentation of Asgard optional dependencies

[bmoyles]

It's not immediately obvious that some functionality is dependent on Eureka (at a minimum)--for example, /applications/list will show 0 instances for applications if Eureka is not deployed and configured. Similar to the conventions issue I opened, it would be awesome to have a page that covered any optional dependencies and what the tradeoffs are when choosing to go without them.

Optionally (and maybe this is worth logging a separate issue), functionality would fall back to some sane default in the absence of things like Eureka (rather than look for discovery instances for a count, simply use the number of instances Running under ASGs associated with the application)

[eciramella]

I really like this - we're looking to use the rest api to query various parts of asgard and currently, we have to slog through json (instead of being able to ask the application directly how many instances it has that it knows about).

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