Add a `paddle usage` command to support collecting demo usage.

* The $TRAVIS_COMMIT_RANGE could be not existed, because the git history
  can be changed and rebased. Here we alway run Travis-CI when `git diff`
  cannot return the modified files.
* Also do not push docs update if is not PaddlePaddle/Paddle.

- Put the 'ARG DEBIAN_FRONTEND=noninteractive' ahead of the first
  'apt-get install' invoke.
- Add comments to Dockerfiles to explain why we're adding Bazel
  installation.
Signed-off-by: NLi Peng <lipeng@unisound.com>

Also add DEBIAN_FRONTEND=noninteractive to suppress the debconf
messages.