Update git submodules

Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Merge "store/load offline docker images"
  - store/load offline docker images

    JIRA: DOVETAIL-423

    usage:
    1.cd ${DOVETAIL_HOME}/dovetail/utils/offline
    2.edit config.yaml as needed
    3.on onsite host, <python download.py>  --> save docker images to defined path
    4.on offline host, <python load.py> ---> load docker images to offline env
    5.this can be easily extended to other images beside docker images

    Change-Id: I97d843e154ecf8d66cafb9ea9594fe73343ee591
    Signed-off-by: MatthewLi <matthew.lijun@huawei.com>