Merge pull request #5 from chengsun/master

Minor cosmetic changes
Don't create Artifact if there were errors

Prevents nil-dereference crash when errors occur
Revert "Add a pause to ensure SSH has a chance to start. This is a tempoary workaround for the fact that the SSH tunnel code will cause packer to halt if the destination does not exist."

This reverts commit fd35d7c9846566f39143972e846c39daac363a05.
Add generic interruptible wait method
Add install_timeout to stop build if step_wait takes too long

Halts the build when the VM does not shutdown within the timeout period
Implemented with interruptible_wait, so step_wait is also interruptible