[feature] Allow specify file for add-ssh-key command

[Erik Lönroth]

Currently, adding a ssh public key to a juju model is very awkward and inexperienced linux users often finds this a difficult and cumbersome activity.

    juju add-ssh-key "$(cat ~/.ssh/id_rsa.pub)"

or
   juju add-ssh-key "ldkfljghkdjfhglkdfjhglskdjfghlskdfjghlskdfjghslkdfjghsldkjfhgsldkjfghsdfg"

... this is not user friendly.

I propose a simple fix to this by allowing for a single file reference instead:

   juju add-ssh-key ./mysskey.pub

This is a trivial fix to make life alot easier. Especially to non experts.