Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "Include global-jjb v0.15.0"
  - Include global-jjb v0.15.0

    Global-JJB[1][2] is a collection of reusable JJB templates created by The
    Linux Foundation Release Engineering team.

    Using it will allow OPNFV to take advantage of the Release Engineering
    work being done across all the other Linux Foundation projects.

    Including the submodule won't affect current jobs as templates aren't
    used until they are listed in a job definition. New releases of
    global-jjb won't affect our jobs since the submodule is pointed at a tag
    and needs to be manually updated to a new release.

    [1] https://github.com/lfit/releng-global-jjb
    [2] http://docs.releng.linuxfoundation.org/projects/global-jjb/en/latest/index.html

    Change-Id: Id1548438b57df177a7a37cb3ed5de6ae0febab3d
    Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>