CI: Configure gitlint for Conventional Commits

Turn on Conventional Commit message subject validation. This will affect
CI specifically, but developers can avoid having CI fail by assuring
that they have installed pre-commit and that they have also run

pre-commit install --hook-type commit-msg

The above is needed because pre-commit does not install commit-msg hooks
by default

Change-Id: If45c7a256f7d769eb092d867217468b9d6c53384
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>