Update git submodules

* Update docs/submodules/sfc from branch 'master'
  - Include the tacker python client to the requirements.txt

    In OPNFV/SFC project we are going to substitute
    the custom tacker that we used so far with the upstream one.

    So as outcome of the aforementioned change we will need the
    upstream tacker client.

    Change-Id: I8425c7168c1be69d090481efbf3f966e8d04e609
    Signed-off-by: Dimitrios Markou <mardim@intracom-telecom.com>

diff --git a/docs/submodules/sfc b/docs/submodules/sfc
index e0f569c..7287446 160000 (submodule)
--- a/docs/submodules/sfc
+++ b/docs/submodules/sfc
@@ -1 +1 @@
-Subproject commit e0f569cd2146951f33207141df4576bf98cd5769
+Subproject commit 7287446920ed8859a7de78eef4235367a48a7d78