Update git submodules

Update git submodules

* Update docs/submodules/snaps from branch 'master'
  - Merge "Apply upper-constraints when testing SNAPS"
  - Apply upper-constraints when testing SNAPS

    It conforms with OpenStack rules [1] and enforces the same testing
    env over the OPNFV projects [2].

    [1] https://wiki.openstack.org/wiki/Requirements
    [2] https://wiki.opnfv.org/display/functest/Requirements+management

    Change-Id: Iad277b305dbb6c9e96894f5809edcb27a750e418
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>