communicator/ssh: ability to use a password-protected key

[GH-266]

/cc @rgarcia

/cc @rasa - I changed up quite a bit here. I tried to reduce function
count if possible, renamed some functions, etc. Overall the
functionality was all spot on, but I felt the functions were too
specialized. Thanks!

builder/amazon/common: Log error messages from goamz on state change errors

/cc @jmassara - easy stuff :)

StepModifyAMIAttributes

/cc @jmassara - I pulled out the AMI stuff into a common config
struct and also added in the new template stuff (that didn't exist
when you made the pull, so not your fault! :))

/cc @jmassara