cmake: install cython modules

cmake: install cython modules

* fix CYTHON_ADD_MODULE() macro. because python_add_module() offered by
  FindPythonLibs.cmake creates a target with name of ${name}, which conflicts
  with existing targets like "rbd" or "rados". so we can not reuse the
  name in ${name}.pyx. and instead, we should specify the target name
  explicitly.
* add distutils_install_cython_module() function to build and install
  cython modules.
* we can split build and install of cython module, but the install phase
  always tries to build the module. so keep it this way. will look at it
  later on.
* move the variables initializations into the Distutils.cmake module.

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