Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'stable/gambia'
  - Fix Undercloud URL path

    Modify the URL path for undercloud disk image to opnfv artifacts. The
    undercloud image was removed from upstream TripleO, so we need to use
    the latest known good version for Gambia release, stored in OPNFV
    artifacts. In the future we will remove downloading undercloud image and
    just use the overcloud image for undercloud.

    Change-Id: I17a3bbaf67d55de05c195712d7fee6e89676bc16
    Signed-off-by: Tim Rozet <trozet@redhat.com>