Better UX when adding new repos to hydra

[angerman]

Currently adding a new repo to hydra for CI involves editing at least three files. Which can be seen for example in #578. This is quite ergonomically and prone to errors (I think).

It would be pretty cool if we had a json file that looked something like this:

[
  ...
  , { "loony tools": "https://github.com/input-output-hk/tools" }
  ...
]

from which this scaffolding that's currently needed would be driven. Maybe optionally have a key bors: true if borsng should be enabled on that repo or not.

[disassembler]

I think we've pretty much optimized this the best we can without altering hydra internals itself. I'm going to close this out. As an example:
https://github.com/input-output-hk/iohk-ops/blob/master/jobsets/default.nix#L128-L134

makes it have flags for bors and prs. spec.json cannot be generated, and we have to have the PrsJson at the top to fill in spec.json. Notifications are also a separate workstream as jobsets are handled at eval time and notifications are built and deployed as a config file hydra parses. Would be nice to clean this up further, but not likely in the short term.