[macosx-*] Improve DRYness & correctness of Mac templates.

[fnichol]

The following improvements are made in this commit:

• Using the template user variable to replace repetition of
  "macosx-10.*" style strings throughout
• Bump the minimum version of Packer from 0.6.0 to 0.7.0
• Supply a HOME_DIR environment variable for provisioner scripts to
  use
• Remove the optional autologin logic, which wasn't being triggered by
  default leading to less behavior "tuning knobs" in the templates
• Remove now-unused user variables arch and autologin
• Add consistent _README user variable to explain how to obtain and
  prepare the base disk image
• Rename all template names from "macosx-10.*" to "chef/macosx-10.*"
• Extract the iso_checksum_type into a user variable and reference it
  in the templates
• Call all provider scripts with sh -eux which will: (e) exit
  immediately if a command exists with a non-zero exit status, (u)
  treats unset variables as a error, and (x) outputs the command and its
  expanded arguments or associated word list
• Ensure that all the provider shell scripts are more conservatively
  bourne shell friendly
• Add a call to VMware's vmware-tools-cli disk shrink command in
  scripts/macosx/minimize.sh if the provider is vmware-based