add inference api demo impl

* Dockerfile: unify dev and product docker environment.

* Fix check style error.

* Use envrioment valuable to get version info.

* use latest pip version

* fix ci

* update by comment

* fix build capi

* update by comment

This reverts commit 0446220e.

Reason:

Rerun failed test hides flaky test.
Flaky test can be bugs, for example, race condition.
Test shouldn't be flaky, if a test is flaky, it should be fixed.

* CI: rerun failed tests.

* fix check style error.

* Build: simplify travis CI script.

* Add linkcheck and run gen_doc in host machine not in docker.

* Add LinkCheck python package.

* Scripts: refactor paddle bash shell scripts.

* Fix indent.

* Modify readme documents.