[centos-*] Improve DRYness, correctness, and speed of CentOS templates.

[fnichol]

The following improvements are made in this commit:

• Ensure that all related provisioner shell scripts are written using
  more conservative bourne code idioms
• Call all provisioner scripts with sh -eux which will (e) exit
  immediately if a command exits with a non-zero exit status, (u) treats
  unset variables as an error, and (x) outputs the command and its
  expanded arguments or associated word list
• Use the template user variable to replace repetition of "centos-*"
  style strings throughout
• Support a HOME_DIR environment variable for provisioner scripts to
  use
• Remove now-unused user variable arch
• Extract the iso_checksum and iso_checksum_type values into user
  variables and reference them in the templates
• Construct the iso_url value from the mirror, mirror_directory,
  and iso_name user variables
• Add a ks_path user variable and use it throughout the templates
• Add an iso_name user variable and use it throughout the templates
• Use the http_proxy, https_proxy, and no_proxy environment
  variables if set on the workstation and make them available to the
  provisioner shell scripts to speed up package installations, etc.

[fnichol]

\cc @cheeseplus

[cheeseplus]

👍

[legal90]

@fnichol Could you please explain why have you replaced all box names with this one?

"box_basename": "__unset_box_basename__",

Now we always have to specify box name explicitly while running packer.