Update git submodules

* Update oslo.middleware from branch 'master'
  - add lower-constraints job
    
    Create a tox environment for running the unit tests against the lower
    bounds of the dependencies.
    
    Create a lower-constraints.txt to be used to enforce the lower bounds
    in those tests.
    
    Add openstack-tox-lower-constraints job to the zuul configuration.
    
    See http://lists.openstack.org/pipermail/openstack-dev/2018-March/128352.html
    for more details.
    
    Change-Id: I477dc48b392c20e93c69464b2b45c21ba7f4805b
    Depends-On: https://review.openstack.org/555034
    Signed-off-by: Doug Hellmann <doug@doughellmann.com>