Add Compose.old_compose_path and use it when searching for files in old compose.
The current code calls `find_old_compose` followed by multiple `os.path.*`
calls to find out if particular file exists in the old compose. This
duplicates code a lot and makes it harder to read.
In this commit, new `Compose.old_compose_path` is introduced and
used instead of direct calls of `find_old_compose`.
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>