Ensure /usr/local/bin is on PATH

For some reason, this isn't on the default PATH. Adding it to the base
box to ensure it becomes available for all users.
Symlink packer in /usr/local/bin

See the comment in the puppet manifest.
Reference packer from /usr/local/bin

This is because on some systems, there is another packer in /usr/sbin.
Symlinking it then referencing it with a full path is easier than
trying to put the application's directory on PATH, which could vary per
installation.