Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Disable restricts of sdnvpn test cases

    1. The original testcases.yaml file of Functest restricts sdnvpn test cases
       to run on Fuel, Apex or Netvirt on bgpvpn scenario.
    https://github.com/opnfv/functest/blob/master/functest/ci/testcases.yaml#L278
    2. Dovetail has a testcases.yaml file to allow sdnvpn test cases run on
       any commercial SUTs.
    https://github.com/opnfv/dovetail/blob/master/etc/userconfig/testcases.yaml#L113

    Change-Id: I0d5ed5b22e55dc3dc0fe584dc57b28239334b7de
    Signed-off-by: xudan <xudan16@huawei.com>

diff --git a/docs/submodules/dovetail b/docs/submodules/dovetail
index 6de223d..b2c513d 160000 (submodule)
--- a/docs/submodules/dovetail
+++ b/docs/submodules/dovetail
@@ -1 +1 @@
-Subproject commit 6de223dad312a4fac4a410fc4a4a21f457c06f27
+Subproject commit b2c513d028451cc142e1647bf749f245020bf15b