Update git submodules

Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  to 9b8d8fcc9f4f191374f730b13fcb0ce18905864c
  - Fault fixing for cmd_exec util method

    When the 'dovetail run' command is assigned, the error
    "TypeError: must be str, not bytes" is appeared.

    Due to a different handling of strings in the Python3, this error
    is appeared during of command execution (i.e. exec_cmd method).

    Each piece of the string should be decoded, before it will be
    appended to the rest one.

    Signed-off-by: Panagiotis Karalis <panos.pkaralis@gmail.com>
    Change-Id: I65629f3f76cc3e44f3926a744d00791ef588d2aa