Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Add core pinning settings for VPP

    Also updated os-net-config to enable VPP service on startup,
    as well as updating show interfaces command.

    JIRA: APEX-407
    os-net-config-pr: 6
    opnfv-tht-pr: 129

    Change-Id: I77c590e966d2532d894ec96bb60ffdf914a90f6d
    Signed-off-by: Feng Pan <fpan@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 8300dc9..bc28dc2 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 8300dc9ab70a051d627c1ed67486a38ee74f213a
+Subproject commit bc28dc258e2852d68a8b16f31903b80d4d6c8e27