Revert "ceph-conf: no admin_socket"

This reverts commit 72715b235a0daee7ab8e5cd3ab6e415de2939df9.

This breaks the ceph cli, which uses ceph-conf --show-config-value ... to
get the admin socket.
Revert "ceph-conf: do not log"

This reverts commit acc31e75a3e7115c00f9980609948455e3b2d49e.