Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Install missing packages on undercloud image

    Currently, tripleo images are not synchronized to CentOS CDN, where
    we download our images. The bug is tracked at:
    https://bugs.centos.org/view.php?id=12994

    This causes our undercloud image to be out of date and missing
    packages which are required during undercloud install. This causes
    puppet to perform yum install of those packages during undercloud
    deployment, which will fail in cases where internet is not accessible
    during deployment.

    While we wait for resolution of CentOS CDN issue, to work around
    this issue, we need to add those packages in undercloud image.

    Change-Id: Ic5a2cfddda544c2d7b6b5bcb145ade8276fd411b
    Signed-off-by: Feng Pan <fpan@redhat.com>