Node verifier should never use ssh agent.

mcollective is a long running process which is run as a daemon. It's never correct for the _server_
to use the user who starts it's agent forwarding to verify things with, as this will stop working
as soon as the user who started the daemon logs out
Be able to interpolate the username into authorized_keys path

We don't want users to be able to add arbitrary ssh keys without oversight,
ergo we push out a set of authorized keys files to machines with puppet
in a directory the user doesn't have write access to.