Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "jjb: xci: Use centos-minimal instead of centos7 element"
  - jjb: xci: Use centos-minimal instead of centos7 element

    The full distro elements are not well maintained. Upstream OpenStack
    also prefers the minimal elements when building images for the CI so
    we should do the same to minimize the delta between the two systems.

    Change-Id: Ifeddb0a74c2434df2cfa95cde36b4aaac1516698
    Signed-off-by: Markos Chandras <mchandras@suse.de>