Update git submodules

Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Add User Guide per installer - APEX

    Trying to install the dovetail project on APEX, some instructions
    have been found that are not clear enough.
    The aim of introduction of new folder and user guide documents for
    each installer is to make a dovetail deployment easier, using tips
    and advices that are applicable for each installer respectively.

    JIRA: DOVETAIL-743

    Change-Id: I907b694d859164031b170cb0c7bb4f69b9ec889b
    Signed-off-by: Panagiotis Karalis <pkaralis@intracom-telecom.com>