Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Merge "baro: update collectd-openstack branch"
  - baro: update collectd-openstack branch

    COLLECTD_OPENSTACK_GIT_BRANCH has been updated to stable/pike so that aodh
    changes will be picked up

    Previously, the branch was not actually being checked out, but was being
    used as a name for a new branch. This behaviour has been corrected.

    Change-Id: I516c4311d2a0cedcca6ec293b62c21234b1e1975
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>