cmake: install python packages into $CMAKE_INSTALL_PREFIX

cmake: install python packages into $CMAKE_INSTALL_PREFIX

* Distutils.cmake:
  set --prefix=${CMAKE_INSTALL_PREFIX} for python packages installed using
  setuptools. it was set to --prefix=/user only when $DESTDIR is set. so
  if user installs ceph using -DCMAKE_INSTALL_PREFIX, these python
  packages still go to /usr, which is unexpected.
* ceph-disk/CMakeLists.txt:
  install script into ${CMAKE_INSTALL_SBINDIR} instead of /usr/sbin

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