Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Update VPP/ODL versions for FDIO scenarios

    Pin the same set of FDIO and OpenDaylight versions for all fdio
    scenarios.

    Note that fdio_netvirt-fdio still uses a customized ODL version.

    JIRA: APEX-451

    Change-Id: Ic5ac5aeead9967fe383447e23653276cde2dfe60
    Signed-off-by: Feng Pan <fpan@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 168e9e9..6557a83 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 168e9e92a39d03b5455938dfa7269017602e99e8
+Subproject commit 6557a832f652ac8a57ef9ad4c7b5cded3f9a7db2