doc,tests: force checkout of submodules

doc,tests: force checkout of submodules

When updating submodules, always checkout even if the HEAD is the
desired commit hash (update --force) to avoid the following:

    * a directory gmock exists in hammer
    * a submodule gmock replaces the directory gmock in master
    * checkout master + submodule update : gmock/.git is created
    * checkout hammer : the gmock directory still contains the .git from
      master because it did not exist at the time and checkout won't
      remove untracked directories
    * checkout master + submodule update : git rev-parse HEAD is
      at the desired commit although the content of the gmock directory
      is from hammer

http://tracker.ceph.com/issues/11157 Fixes: #11157

Signed-off-by: Loic Dachary <ldachary@redhat.com>