Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Moves RDO images URI to working server

    CBS has been broken and the overcloud and undercloud artifacts are not
    getting updated there.  The new URI is a workaround because it is a
    single file server acting as a backup until the CBS repo is fixed.

    JIRA: APEX-458

    Change-Id: I743747015d69b17d75aa7b231bc07c935788ea20
    Signed-off-by: Tim Rozet <trozet@redhat.com>
    (cherry picked from commit ef3dc1ce0323fa0881e416cd4b9028fb4250b719)