Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Remove qemu install

    qemu-ev is now included in overcloud.

    Change-Id: I0d62793d267465c99fde489b8d39c27a87a20a4c
    Signed-off-by: Feng Pan <fpan@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 5d52dc8..8a973df 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 5d52dc853acabfedf5d52130991650920832e801
+Subproject commit 8a973df23bf27f14c601100b63fa9f8ccbee0b76