Use git submodule update --init --recursive

In a case of nested submodules, like swagger-ui inside mgmt inside osv,
only the --recursive update all the submodules, all the way down.

Signed-off-by: Tzach Livyatan <tzach@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>