Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Pin puppet-opendaylight for puppet 3.8 compatibility

    Change-Id: Ib9052fafd601c7b269f0f6a3e98d83e63b07f12f
    Signed-off-by: Feng Pan <fpan@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index fdd6078..59450b3 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit fdd607899785ff8f3f838fcf744c79786f5349b0
+Subproject commit 59450b35d3fddded17b125da4c82f8153585f16d