Update git submodules

* Update docs/submodules/apex from branch 'stable/fraser'
  - Updates undercloud/centos repo during build phase

    Some dependencies are failing to install for undercloud install because
    the repo has expired on the frozen stable/fraser image.  The kernel
    devel/headers packages are missing in the centos updates repo, so need
    to specify their location.

    Change-Id: I878323b01b624c452a06cf5fde40eaae196d91bd
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 283bb98..0b57899 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 283bb98ae05b77979f00716d3c9cd3fff6af3651
+Subproject commit 0b5789905915f0468d18aa3c42eb469a331e8315