config: Add a per-name default keyring to front of keyring search path.

config: Add a per-name default keyring to front of keyring search path.

This lets us have e.g. /etc/ceph/ceph.client.admin.keyring that is
owned by root:admin and mode u=rw,g=r,o= without making every non-root
run of the command line tools complain and fail.

This is what the Chef cookbook has been doing for a while already.

Signed-off-by: Tommi Virtanen <tv@inktank.com>