cmake: rearrange configure_file()

cmake: rearrange configure_file()

AddCephTest and googletest's CMake scripts also call
find_package(Python3...), but they do not specify the required minor
version of Python3. by default, find_package(Python3...) picks the highest
available python3. so, if we have multiple python3 versions installed in the
system, and the highest python3 version is not the one specified by the
-DWITH_PYTHON3=3.x.y in the cmake command line, we might end up using a
different python3 for the ceph CLI. and even worse, the required python3
package might not available for the picked python3 interpreter found by
googletest. as, in general, only a single python3 has the full access to
prepackaged python3-* shipped by a GNU/Linux distro.

in this change, the configure_file() calls are rearranged to the top of
src/CMakeLists.txt, so they have less chance to use the "polluted" cmake
variable for their subvars.

this change address the test failure where we have, for instance, python3.8
installed on RHEL8/CentOS8, where python3.6 is the python3 which has
the access to the python3-* packages.

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