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

there is a other doc file for this called salt.html.markdown which contain a better history

[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 :)