Add support for custom ssh key

[Hakihiro]

Actually we can add it to the user parameter.

Eg :

servers: [{
            user: "-i {{pathToPem}} user",
            host: "myhost",
            to: "/toto"
        }]

But it can be more userfriendly if we can do:

servers: [{
            user: "user",
            key: "{{pathToPem}}"
            host: "myhost",
            to: "/toto"
        }]