manifest: add submanifest.default_groups attribute

When the user does not specify any manifest groups, this allows the
parent manifest to indicate which manifest groups should be used for
syncing the submanifest.

Change-Id: I88806ed35013d13dd2ab3cd245fcd4f9061112c4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/335474


Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>