Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'master'
  - baro: updating refs to collectd-ceilometer

    Since Ceilometer support was removed from the
    collectd-ceilometer-plugin[1], the repo name and ceiloemter are
    misleading. A change was submitted to openstack-infra to rename
    the repo [2]

    This change removes references to collectd-ceilometer from apex.
    These changes are independant of the repo rename.

    [1] https://review.openstack.org/#/c/490823/
    [2] https://review.openstack.org/#/c/500768/

    Depends-On: I6cf22c081aa1e40f64995be40b6225855faf4092
    Change-Id: Ib3a7766e28fd03a86a995eea4b9abc996498200e
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>