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.
* Add new functions to communicator helper to return the user, password, host, based on the communicator used.
This implementation can help then generalize the provisioeners later on.
* Update null builder checks to utilize the new functions and check for ANY hostname or user or password
* Update builder to user any hostname
WinRM exposes an HTTP transport decorator that can be used for different
authentication schemes. Windows on Azures requires this if one is to use
the out of the box configuration.
