Add multi sources support

[olblak]

This PR comes hand in hand with #181 to solve #158
While keeping backward compatibility, I am switching from one source to a map of sources.
Now for each condition/target, we must specify the sourceID parameter to specify which source a condition/target depends on,
it mainly used to get default values for those stages.

An example of this change

---
title: Bump jenkinsciinfra/helmfile image
sources:
  helm:
    name: Get Latest helm release version
    kind: githubRelease
    spec:
      owner: "helm"
      repository: "helm"
      token: {{ requiredEnv .github.token }}
      username: olblak
      version: latest
  helm2:
    name: Get Latest helm release version
    kind: githubRelease
    spec:
      owner: "helm"
      repository: "helm"
      token: {{ requiredEnv .github.token }}
      username: olblak
      version: latest
conditions:
  isENVSet:

[olblak]

I modified this PR to only focus on multi sources and remove any mention of more advanced templating.