Update git submodules

Update git submodules

* Update docs/submodules/sdnvpn from branch 'stable/euphrates'
  - Updating docs for Euphrates

    Moved previous user guide content into the right places,
    updating the section structure along the way.

    Started to edit the release notes but this needs more work.
    This will be merged now to enable link checking by the docs team.
    Remaining chan ges to be done in a separate review.

    JIRA: SDNVPN-185

    Change-Id: Idd128b05f8ceb7195086b813d499b3f12243d025
    Signed-off-by: Tim Irnich <tim.irnich@ericsson.com>
    (cherry picked from commit 8e1aee4da772dec5f95bd50611323289eee35628)