Add new key 'repo' to allow specifying multiple repos as the source
repositories. And change 'source_repo_from' to allow specifying multiple
vairant names to use variant repos as source repositories.
Doc of 'source_repo_from' is updated to not mention URL is supported,
though we still support that in code. User should add url of repos in
'repo' key instead of 'source_repo_from'.
Fixes: #508
Signed-off-by: Qixiang Wan <qwan@redhat.com>