Automagically setup submodules on first run.

I mean, who's reading the README, anyway?

Signed-off-by: Thorsten Behrens <tbehrens@suse.com>
Merge pull request #2069 from thorstenb/thb-buildconvenience-1

Automagically setup submodules on first run.

Reviewed-by: Sage Weil <sage@redhat.com>