cmake: drop WITH_PYTHON2 option

cmake: drop WITH_PYTHON2 option

* drop WITH_PYTHON2 option
* print warning message if WITH_PYTHON3 is disabled
* drop MGR_PYTHON_VERSION option, as we don't support use different
  python version for python binding and ceph-mgr embedded python
  interpreter anymore. as before switching to python3-only build,
  we can build python3 and python2 python bindings, and ceph-mgr
  can use either of them. but after switching to python3-only
  build, ceph-mgr has to use whatever python version used to
  build python binding.
* move WITH_PYTHON3 option to $top_srcdir/CMakeLists.txt, as ceph-mgr
  and python binding will share this option.
* hardware ${PYTHON_VERSION} to 3
* hardware ${Python${PYTHON_VERSION}_VERSION_MAJOR} to 3
* only build boost library with python3
* s/Python_EXECUTABLE/Python3_EXECUTABLE/
* update the build scripts and packagings accordingly
* rename all cython${PYTHON_VERSION}_* targets to cython_*
* update distutils_install_module() so it does not take python_version
  parameter anymore.

Signed-off-by: Kefu Chai <kchai@redhat.com>