Flatten checks catalog ouptut

[arbulu89]

In order to avoid doing "complex" operations in golang, as we can make them in elixir, which is more suitable for that, thie PR flattens out the checks catalog output, so we just return a list with all the checks.

The work of transforming this content in something the frontend consumes will be done by the server side.

Something like:

[
  {
    "id": "156F64",
    "name": "1.1.1",
    "group": "Corosync",
    "provider": "azure",
    ...
  },
  {
    "id": "53D035",
    "name": "1.1.1.runtime",
    "group": "Corosync",
    "provider": "azure",
    ...
  },
  {
    "id": "A1244C",
    "name": "1.1.2",
    "group": "Corosync",
    "provider": "azure",
   ...
  },
  {
    "id": "FB0E0D",
    "name": "1.1.2.runtime",
    "group": "Corosync",
    "provider": "azure",
    ...
  },
  {
    "id": "845CC9",
    ...

[nelsonkopliku]

LGTM