Update git submodules

Update git submodules

* Update nova from branch 'master'
  to 4b71dab239b4f82d50297ec5bd887c207b43b7ea
  - Merge "Remove tpool_execute as it is unused"
  - Remove tpool_execute as it is unused

    The previous commits removed all the users for nova.utils.tpool_execute
    so this patch removes that utility.

    This also allows us to inline the _pass_context helper as now it is only
    used by spawn_on.

    Change-Id: I8520ccf4b5526543681c8c3864aaeef501462e9e
    Signed-off-by: Balazs Gibizer <gibi@redhat.com>