Update git submodules

* Update docs/submodules/apex from branch 'stable/fraser'
  - Merge "Avoiding exception if guest refuses to detach a volume" into stable/fraser
  - Avoiding exception if guest refuses to detach a volume

      https://review.openstack.org/#/c/551950/

    JIRA: APEX-589

    Change-Id: I490f1030a32a44316e62c93cf1ed8642aef30527
    Signed-off-by: Ricardo Noriega <rnoriega@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 4b8f1b1..4dff434 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 4b8f1b163570256eee7cf69f4de61c7c6a84046c
+Subproject commit 4dff434d9c16b3d2bd4942fbc5769ac406f6d072