cluster kick: 2 issues: PEM not picked up, error on SSH error

Hi,

While using your ironfan gem, we have identified a couple of issues with cluster kick:

 - it does not seem to pick up the pem file automatically
 - if an SSH error occurs, an exception is thrown

The fixes:

 - for the pem issue: if `config[:identity_file]` is missing, set to `"#{Chef::Config.ec2_key_dir.to_s}/#{@name_args[0]}.pem"`
 - SSH error issue: before calling `session_from_list` set `@action_nodes`, see https://github.com/opscode/chef/blob/master/lib/chef/knife/ssh.rb#L141

Hopefully this helps.