SSH config: command for bastion server proxying

Host 51.21.21.11
  IdentityFile ~/.ssh/mykey.pem
  User ec2-user
  ForwardAgent yes

Host 10.*
  ProxyCommand ssh ec2-user@51.21.21.11 nc %h 22
  User ec2-user
  IdentityFile ~/.ssh/mykey.pem
  ForwardAgent yes

Put your ssh public key in ~/.ssh/authorized_keys along the path.