ceph-post-file: single command to upload a file to cephdrop

Use sftp to upload to a directory that only this user and ceph devs can
access.

Distribute an ssh key to connect to the account.  This will let us revoke
the key in the future if we feel the need.  Also distribute a known_hosts
file so that users have some confidence that they are connecting to the
real ceph drop account and not some third party.

Signed-off-by: Sage Weil <sage@inktank.com>