Update git submodules

Update git submodules

* Update docs/submodules/fuel from branch 'stable/euphrates'
  - states: maas: Stop using maas-stable PPA

    Currently, Xenial repos provide MaaS 2.2.x, while the PPA bumped it
    to 2.3.x. Since we switched to 2.3, we observed a rare wrongful state
    transition from 'Deploying' back to 'Ready'.
    Drop the PPA, falling back to 2.2 from mainline distro repos.

    JIRA: FUEL-312

    Change-Id: I3daa118059f37cbeca076da685661c28f3a28a97
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
    (cherry picked from commit 9da33bc85d681950a09452f28ca39df2108b0b56)