Make JSON endpoints configurable

[jlebon]

I'd like to be able to write upgrade tests that require talking to a local dumnati server serving test-specific metadata. Right now, the releases.json and ${stream}.json is hardcoded into the app. Can we make these be configurable somehow? I see it doesn't use config files yet, and maybe we don't want to go all the way there until we sort out the merger with Cincinnati -- maybe just env vars for now would work?

[jlebon]

I'd like to be able to write upgrade tests that require talking to a local dumnati server serving test-specific metadata.

Hmm, actually... I guess I could hack this up without using dumnati at all by just serving the JSON at the URL zincati will hit. A bit underhanded, and implies it's not entirely testing zincati itself, though it simplifies the setup quite a bit.

[lucab]

I've left a TODO to figure out the configuration story:

fedora-coreos-cincinnati/dumnati/src/main.rs

Line 82 in b69bc52

// TODO(lucab): figure out all configuration params.