[#731] Use AuthorizedKeysCommand instead of replicating ~/.ssh/authorized_keys on the filesystem

The new sshd_config AuthorizedKeysCommand is present officially in OpenSSH 6.6, Debian Wheezy backports, CentOS 6 (early redhat-specific patch), so about everywhere.

This will make user ssh keys uploads effective immediately.

Documentation:

 AuthorizedKeysCommand
         Specifies a program to be used to look up the user's public keys.
         The program must be owned by root and not writable by group or
         others.  It will be invoked with a single argument of the user‐
         name being authenticated, and should produce on standard output
         zero or more lines of authorized_keys output (see AUTHORIZED_KEYS
         in sshd(8)).  If a key supplied by AuthorizedKeysCommand does not
         successfully authenticate and authorize the user then public key
         authentication continues using the usual AuthorizedKeysFile
         files.  By default, no AuthorizedKeysCommand is run.

 AuthorizedKeysCommandUser  # AuthorizedKeysCommanRunAs on CentOS
         Specifies the user under whose account the AuthorizedKeysCommand
         is run.  It is recommended to use a dedicated user that has no
         other role on the host than running authorized keys commands.