[juju authorised-keys add] cannot specify a key file

[Peter Matulis]

This should work and is what most users expect to be able to do:

$ juju authorised-keys add key-file
cannot add key "key-file": invalid ssh key: key-file

What currently needs to be done is this:

$ juju authorised-keys add $(cat key-file)

Please allow users to specify a key file instead of using the above hack, or worse, pasting the key itself as some novice Linux users may do (if they ever figure out what's wrong).