Update git submodules

* Update docs/submodules/apex from branch 'stable/euphrates'
  - Updating the iso build to CentOS 7.4

    JIRA: APEX-522

    Change-Id: I03efc6ccad708d12f3fdabd9ace9935edb31d4bb
    Signed-off-by: Dan Radez <dradez@redhat.com>
    (cherry picked from commit 1f8cf744499c5a97c88c02e949593c5bb203e2b0)

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 2ee71b2..15208f5 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 2ee71b201c0a607f1814bf5fe3e6971f6357db42
+Subproject commit 15208f52f1c9dbea048cd363e0caae3a8e95d954