os/FileStore: include filestore backend, f_type in osd metadata

Report the FileStore backend and underlying fs type as part of the osd
metadata shared with the monitor.

Signed-off-by: Sage Weil <sage@inktank.com>
os: ObjectStore::collect_metadata unit tests

And an emacs-executable reminder about how to run a specific test.

Signed-off-by: Loic Dachary <loic@dachary.org>
os: FileStore::create_backend unit tests

Assert that the fs_type is mapped to a backend that claims to have the
matching name.

Signed-off-by: Loic Dachary <loic@dachary.org>