Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Merge "Bump bifrost sha and set UPPER_CONSTRAINTS_FILE"
  - Bump bifrost sha and set UPPER_CONSTRAINTS_FILE

    The lack of UC means that pip is attempting to install libvirt-python 4.1.0,
    which is failing on Ubuntu due to
        https://bugs.launchpad.net/openstack-requirements/+bug/1753539.
    Currently upper-constraints limits libvirt-python to 4.0.0, which does not
    have this issue.

    Change-Id: Idc97a0d9928cebafc9167614723236d2584cc93f
    Signed-off-by: Fatih Degirmenci <fdegir@gmail.com>

diff --git a/docs/submodules/releng-xci b/docs/submodules/releng-xci
index 92f89a5..c4651cc 160000 (submodule)
--- a/docs/submodules/releng-xci
+++ b/docs/submodules/releng-xci
@@ -1 +1 @@
-Subproject commit 92f89a5d2028675cf03d8b828da1fd7233f77115
+Subproject commit c4651cc986ac5277d95027df77bc42cc783f2777