scms: MultiSCM is generated for scms hidden in macro

This patch fixes problem with generating MultiSCM in case that scms are
encapsulated in macro.
This patch includes simple tests to cover if-branches for this change.

Example:

- scm:
  name: xxx
  scm:
    - git: ...

- scm:
  name: yyy
  scm:
    - git: ...

- scm:
  name: xxx-and-yyy
  scm:
    - xxx
    - yyy

- job:
  name: my-job
  scm:
    - xxx-and-yyy

Change-Id: If3e7b3d19e51545a5623ee060fb7673a7a989233
Signed-off-by: Lukas Bednar <lbednar@redhat.com>
Closes-bug: #1285515