Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Fixes missing openstack-nova-compute on undercloud

    openstack-nova-compute was missing on the undercloud image and it is
    also missing in the delorean repo from which we froze artifacts for
    OPNFV release.  This is likely due to the age of the repo and to fix it
    we need to update the delorean repo on undercloud and install the
    missing package during build time.

    JIRA: APEX-538

    Change-Id: I0f7cd26d5c8e4ad538b3ba3725ab5e7684a50b6a
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 577ccdd..3c7556e 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 577ccdd1e7ce654ebc7479d8bd1e232abb922571
+Subproject commit 3c7556eb0734706f28588fb952eedea2d424c6d2