Support for adding composer sources

[sabre1041]

Support for adding composer sources to resolve #215

Example Helm values

---
application-manager:
  charts:
    # Top Level RFE App of App Chart
    rfe-automation:
      values:
        charts:
          # RFE App of App Chart
          rfe:
            values:
              charts:
                # Image Builder Chart
                image-builder-vm:
                  name: image-builder-vm
                  values:
                    imageBuilderVM:
                      composer:
                        sources:
                          - id: "transmission"
                            name: "Transmission"
                            type: "yum-baseurl"
                            url: "http://cdn.redhat.edge-lab.net/content/rpms/rhel/8"
                            check_gpg: false
                            check_ssl: false
                            system: false

[jordigilh]

LGTM