Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Merge "xipv6 testcase adjustment"
  - xipv6 testcase adjustment

    JIRA: DOVETAIL-418

    1) ipv6 tc002-tc005 error fix, delete wrong testcases
    2) ipv6 tc018-tc025 os-nosdn-nofeature scenario dependent, make
       it known to the users

    Change-Id: I8e63131b795403ee63566ae59bf46782524acd5a
    Signed-off-by: MatthewLi <matthew.lijun@huawei.com>

diff --git a/docs/submodules/dovetail b/docs/submodules/dovetail
index 3aeab49..fdc6128 160000 (submodule)
--- a/docs/submodules/dovetail
+++ b/docs/submodules/dovetail
@@ -1 +1 @@
-Subproject commit 3aeab49bb64c6c89da37f3d03f781aec85f39015
+Subproject commit fdc6128e73f6fb109673a5b522f33879ca749554