capistrano3でssh agent forwarding ref: http://qiita.com/NewGyu/items/285d6d5152e01f0b9497

file1.txt

$ ssh-add -l
2048 f7:07:18:66:97:f0:8f:7b:40:6f:0b:97:98:de:5e:18 /home/newgyu/.ssh/y.pem (RSA)

file2.txt

$ ssh-add /home/newgyu/.ssh/y.pem

production.rb

set :ssh_options, {
  user: 'ec2-user',
  keys: %w{~/.ssh/x.pem ~/.ssh/y.pem},
  forward_agent: true,
  auth_methods: %w{publickey}
}