Move manifest config logic into ManifestProject

Use ManifestProject properties for config values.

Change-Id: Ib4ad90b0d9a089916e35615b8058942e6d01dc04
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334519
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>