Update git submodules

* Update stevedore from branch 'master'
  to 5189992d719ad15e0e3504947895bf5ba9dc7a1d
  - Order old importlib-metadata results by group
    
    A Debian Buster-based zuul-jobs test started failing when using stestr
    recently [1].  Upon further investigation, this is a Python 3.7
    environment which is affected by a recent breaking change to
    importlib_metadata.
    
    It seems stevedore worked around this with
    Ib9c2b0a14edea91e97d122d2ac93b650029f918e, which was released with
    3.5.1 -- but I was still seeing the issue.
    
    Upon further investigation, the "real_groups" dict being returned here
    with importlib-metadata 4.12.0 is in buckets by group, e.g.
    
     {'group.one': [EntryPoint(name='foo', ... , group='group.one'),
                    EntryPoint(name='bar', ... , group='group.one')],
      'group.two': [EntryPoint(name='moo', ... , group='group.two'),
                    EntryPoint(name='goo', ... , group='group.two')],
     }
    
    This current code seems to return a dict with entry-points by thier
    name, e.g.
    
     {'foo': EntryPoint(name='foo', ... , group='group.one),
      'bar': EntryPoint(name='bar', ... , group='group.one),
      'moo': EntryPoint(name='moo', ... , group='group.two),
      'goo': EntryPoint(name='goo', ... , group='group.two)
     }
    
    This reorgansies the fixup routine to put entry-points in a bucket by
    their group.  With this change, stestr is again finding it's command
    plugins.
    
    [1] mtreinish/stestr#336
    [2] python/importlib_metadata#409
    
    Change-Id: I3496ab1dfa312b1098a869cdfd9a0c6f81653b28