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

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