* add cmake support for ccache
* cleanup: use --env-file for docker run invocations
* make python build compatible with using ccache
* enable building using ccache in selected kokoro jobs
* print ccache stats and the end of run_tests.py
The source code is moved from src/python to
src/python/src. A setup.py is added at
src/python. The build_python.sh and
run_python.sh scripts are updated to build
and run the Python tests by building a
package and installing it in the developer's
Python 2.7 virtual environment.