Update git submodules

* Update docs/submodules/apex from branch 'stable/euphrates'
  - Increase overcloud qcow2 size by 1500mb

    We are seeing build failures with 1200mb qcow2 size expansion, increasing
    to 1500mb.

    Change-Id: Ib177525559c7905c78fe5931c172fce01846b4c3
    Signed-off-by: Feng Pan <fpan@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index ba982af..7281622 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit ba982af00b7ae7901eeef44260f62867dc60efc1
+Subproject commit 72816226723ae708b0611fd58b6274524d7240d0