Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Merge "xci: lib-install: Clear XCI_CACHE prior to downloading openstack-ansible-tests"
  - xci: lib-install: Clear XCI_CACHE prior to downloading openstack-ansible-tests

    If we don't clear the cache, the script errors out with the following:
    fatal: destination path '/root/releng-xci/.cache/repos/openstack-ansible-tests'
    already exists and is not an empty directory

    Change-Id: I22111ed655a4c6790f808155968deb2fd3828e41
    Signed-off-by: Taseer Ahmed <taseer94@gmail.com>