Update git submodules

* Update docs/submodules/functest from branch 'stable/danube'
  - Update the link to firewall_block_image.img

    As the link http://205.177.226.237:9999/onosfw/firewall_block_image.img
    has been deprecated, now the valid link is updated, aiming at running the
    testcase onos_sfc.

    JIRA: FUNCTEST-820

    Change-Id: Id848e3ab92c3b0fb47e3c755d203844dd94d8408
    Signed-off-by: Linda Wang <wangwulin@huawei.com>

diff --git a/docs/submodules/functest b/docs/submodules/functest
index f31cf86..72a840c 160000 (submodule)
--- a/docs/submodules/functest
+++ b/docs/submodules/functest
@@ -1 +1 @@
-Subproject commit f31cf86c8580468ea1b2252be95d41d12b7100db
+Subproject commit 72a840c19aef8d53e72c5bb75a9c6b1398eee29e