cmake: differentiate WITH_CEPHFS from WITH_LIBCEPHFS

cmake: differentiate WITH_CEPHFS from WITH_LIBCEPHFS

WITH_CEPHFS is used for enabling the server side of cephfs, i.e., mds,
while WITH_LIBCEPHFS is for the client side of it, namely libcephfs
and its python bindings. since we have these two different options,
in theory, user is allowed to build with WITH_CEPHFS=ON and
WITH_LIBCEPHFS=OFF, and in that case, `vstart` should not depend on
the non-existent target of cephfs and cython_cephfs. the same applies to
the build of cython_cephfs. this is an unusual combination, but
it is a valid one.

in this change, we

* build the python binding only if WITH_LIBCEPHFS is ON
* add cephfs and cython_cephfs as dependencies of vstart only if
  WITH_LIBCEPHFS is ON

Signed-off-by: Kefu Chai <tchaikov@gmail.com>