Update git submodules

* Update docs/submodules/functest from branch 'stable/danube'
  - Switch to mitaka-eol tag in networking-bgpvpn

    Mitaka is EOL [1] and the Openstack practice is to remove the
    corresponding stable branch.

    [1] https://releases.openstack.org

    Change-Id: Iba37e8839435fa645d0c05bfc4a5453b3da87686
    Signed-off-by: jose.lausuch <jose.lausuch@ericsson.com>

diff --git a/docs/submodules/functest b/docs/submodules/functest
index 72a840c..3b87204 160000 (submodule)
--- a/docs/submodules/functest
+++ b/docs/submodules/functest
@@ -1 +1 @@
-Subproject commit 72a840c19aef8d53e72c5bb75a9c6b1398eee29e
+Subproject commit 3b87204813a4134906131ce8c0fc24a4c54bae71