This commit adds an option to use the local SSH Agent to authenticate
connections to source instances started by the the EBS and Instance
Store builders.
This is of use when the source AMI _already_ has configuration for
authorized SSH keys - for example if one uses an SSH certificate
authority.
A further extension (not implemented in this commit) is to allow SSH
agent use with a pre-defined key pair, in order to allow keys with
passphrases to be used without giving the passphrase to Packer.
7425fef2c7