os/bluestore/bluestore_tool: use boost::filesystem as an alternative

os/bluestore/bluestore_tool: use boost::filesystem as an alternative

the libstdc++ shipped with GCC 7.5 does not have good support of
std::filesystem, among other things, it does not offer
std::filesystem::weakly_canonical(). but boost::filesystem does.
and boost::filesystem is compatible with std::filesystem to some
degree. so let's use it if <filesystem> is not available, we can
take it as a signal that std::filesystem is not quite ready yet.

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