CHEF-2418: rename ssh_password to ssh_password_ng

- commands like knife bootstrap and knife * server create will not set
- can use #has_key? as a test if we're running knife ssh or not
- do not prompt for password if ssh_password_ng does not exist
- not surfaced in knife.rb still uses old ssh_password config option
add tests and moar comment
Merge pull request #1218 from opscode/lcg/save-CHEF-2418

WIP: attempt to save CHEF-2418