Update git submodules

Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Adding reference to EXTERNAL_NETWORK to user guide

    Adding a reference to the EXTERNAL_NETWORK environment variable to the
    user guide so that users add this to their env_config.sh. This variable
    is required, among others, to overwrite the default external network in
    Yarstick.

    Change-Id: I50e609a8c6e20b1dd01c61d5d950ffba792e98fa
    Signed-off-by: Georg Kunz <georg.kunz@ericsson.com>