Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Fix broken build dependency

    libguestfs-tools is failing to install because libvirt-client is
    installed and there is a package conflict.  Removing libvirt-client from
    image before installing libguestfs-tools.

    NSH build is failing in overcloud because the CentOS repos updated with
    newer kernel source and headers, and we are still using older kernel.
    This patch adds the old kernel source and headers.  Upgrading to new
    kernel causes NSH OVS to fail when compiling.

    Change-Id: Ifd500ad138116cfecafb2268ccb580bd44d5efd5
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 7b3a845..2e6b2fa 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 7b3a8455baa35bdde4d7e078d88f9b17a6df9195
+Subproject commit 2e6b2fa375c2356f8af894c88f23d9738d8cf9c0