ceph: fix build and cleanup expression

correct platforms. ceph currently doesn't build on aarch64-linux. So
let's not lie in meta.platforms.

ceph: fix multiple output
We currently just move $out/share/ceph/mgr to
$lib/lib/ceph, and then remove all references to $out with a find
command.

I checked $out, the only reference to $out is in
$lib/lib/ceph/libceph-common.so.0, coming from src/common/options.cc:
https://github.com/ceph/ceph/blob/master/src/common/options.cc#L5050:

>  Option("mgr_module_path", Option::TYPE_STR, Option::LEVEL_ADVANCED)
>  .set_default(CEPH_DATADIR "/mgr")
>  .add_service("mgr")
>  .set_description("Filesystem path to manager modules."),

Just removing the reference might break some behaviour - it should point
to $lib/ceph/mgr instead.

We can fix this in a much more elegant fashion by just passing a custom
CMAKE_INSTALL_DATADIR to the build system.

ceph: fix outdated Boost::python substitutions

Instead of substituting in CMakeLists.txt files, one now needs to set
MGR_PYTHON_VERSION.

ceph: clean up PYTHONPATH wrapping

Set `pythonPath` instead of exporting PYTHONPATH.

Use `toPythonPath` to construct the PYTHONPATH where we need manual
wrapping. There's no ceph-volume, only ceph-mgr.

ceph: set doCheck = false explicitly

and describe why.

ceph: patch more shebangs

ceph: use system-provided gtest and rocksdb