Update git submodules

* Update docs/submodules/apex from branch 'stable/euphrates'
  - Fix ovs-dpdk failures

    - Add quotes to ovs config options in opnfv-environment.yaml
    - Fixed an error where we don't populate kernel paramaters when ovs-dpdk is enabled

    Change-Id: I7579358411a844e9d0c34ca53701f12f46ef7c13
    Signed-off-by: Feng Pan <fpan@redhat.com>
    (cherry picked from commit f1c462054fe302ab6cbf170ab71c0c8edfba680a)

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 6a2bb6d..991531f 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 6a2bb6d1e625e6f58ab8f39c9db1c546a41c921d
+Subproject commit 991531fc2b0f71705962c7cf1fe467e03bab4aa7