Fix erb template.

This fixes the bug introduced in commit d7be61a73bf2e7f9c39b3004500336d111085613
where the package_url erb template variable was refactored
to the top of the file. This causes that the arguments
like ${platform} is evaluated before the platform
variable is populated inside install_chef() function
Do not print AWS secret key on --help and on confirm

This patch hides the AWS secret key so that its not printed
to stdout. This prevents accidents where you paste the program
output to pastebin/gist/forums (been there, done that)
Merge pull request #44 from Applifier/fix/erb-template

Fix erb template.
Merge pull request #45 from Applifier/feature/hide-aws-secret-key

Do not print AWS secret key on --help and on confirm
Pull from upstream

Merge branch 'master' of github.com:airbnb/stemcell