feature request: allow connection using ssh keys

[gilesknap]

I'm automating a cluster build using ansible. The workflow would be cleaner if tpi could use ssh keys instead of requiring a password.

In fact I can use ssh keys for all of the commands that are able to run on the BMC itself e.g. in the following ansible uses ssh to connect to the BMC.

- name: turn off node before flashing
  raw: tpi power off -n {{ node.slot_num }}

But I'm stuck running the flash command on my local host because of disk space constraints and it creates minor security difficulties that in this one case I need to supply a password.