builder/amazon/common: added passing UserData to goamz.

/cc @rgarcia

This adds the -machine-readable flag to Packer which turns all output
into machine-readable format. This is documented within the website
source.

provisioner/salt-masterless: `local_state_tree` should not be mandatory

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

Update and rename salt.html.markdown to salt-masterless.html.markdown
salt.html.markdown had a better history and formatting
And remove local_state_tree from required option from code and docs