Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Add a function to check the cacert file

    1. Currently it won't pass the cacert file to other containers if the
       OS_AUTH_URL is http not https.
    2. However, even though the OS_AUTH_URL is http, it still needs cacert file
       somewhere for Functest and Rally.
    3. This problem is found on Fuel Euphrates.
    4. Add a function to check whether the cacert file exists.
    5. If exists, pass it to testing project containers.

    JIRA: DOVETAIL-616

    Change-Id: Ied7bcc72e8f1c738bbce32c18096ca13641d3cd7
    Signed-off-by: xudan <xudan16@huawei.com>

diff --git a/docs/submodules/dovetail b/docs/submodules/dovetail
index 18f117c..51a7666 160000 (submodule)
--- a/docs/submodules/dovetail
+++ b/docs/submodules/dovetail
@@ -1 +1 @@
-Subproject commit 18f117ce3b88ef626f6de5e5037e220be0f0f0ac
+Subproject commit 51a7666d0d50d0ed13c556d97aded50a3e7f30de