Support for ssh_options[:keys] being an array or a string

Thank you for this tool, ` cap ec2:ssh ` has saved me from a number of unreliable and ever changing bash aliases.

This change add support for arrays in ` ssh_options[:keys] ` by checking the type of ` :keys ` and acting accordingly. By default Capistrano treats ` ssh_options[:keys] ` as an array from to its reliance on Net::SSH.

Thanks again.