VPC instance support

- non VPC
 - (same as before)
- VPC
 - apend bastion server information to .ec2ssh
  - example
````
path: /path/to/ssh_config
aws_keys:
  default:
    access_key_id: YOUR_ACCESS_KEY_ID
    secret_access_key: YOUR_SECRET_ACCESS_KEY
regions:
  - ap-northeast-1
ssh_config_keywords:
  - ProxyCommand ssh -o ServerAliveInterval=60 YOUR_BASTION_SERVER nc %h %p
````
 - Execute ec2ssh update with "--dns-name-key private_dns_name" option