Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  to 51bdc895b22cf8363683919b0b1fc65a641034e5
  - fuel: Disable all CI jobs

    Since Fuel is not going to participate in the next OPNFV release,
    we should free the CI resources by disabling all the related jobs:
    - RTD documentation building;
    - Fuel Master/MaaS Docker image building;
    - verify jobs for patch submissions;
    - daily virtual/baremetal deployment and testing jobs;

    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
    Change-Id: Ic6fa066e719f31e7d97725e815e2dafa22a7d96a