Cygwin setup improvements

Combined the best bits of the improvements suggested by
Ross Smith II <ross@smithii.com> and Joe Fitzgerald <joe.fitzgerald@emc.com>
(via https://github.com/joefitzgerald/packer-windows):

Changelog:

- Removed package dependencies, as Cygwin setup 2.830 no longer requires them to be listed.
- Changed mirror to http://mirrors.kernel.org/sourceware/cygwin/ for speed.
- Replaced hardcoded C: drive references with %SYSTEMDRIVE%.
- Hardcoded architecture to "x86" as "x86_64" seems a bit buggy still, except
  for Windows 2012/2012R2 templates on which 32-bit version seems to crash
  a lot)
- Use Powershell extensions instead of the now deprecated bitsadmin to download remote files
- Changed temporary directory from C:\WINDOWS\temp to %TEMP%.
- Added additional comments
- Split monolithic postinstall script into smaller, more reusable .BAT scripts (which are more appropriate to the Windows platform than Unix-style .sh shell scripts)

Tested these changes, and the boxes

````
win2008r2-datacenter
win2008r2-enterprise
win2008r2-standard
win2008r2-web
win2012-datacenter
win2012-stnadard
win2012r2-datacenter
win2012r2-standard
win7x64-enterprise
win7x64-pro
win7x86-enterprise
win7x86-pro
win81x64-enterprise
win81x64-pro
win8x64-enterprise
win8x86-enterprise
````

successfully install OpenSSH, and VMWare Tools.
Symlink Ruby 1.8 to 2.0.0
Permit optional install of chef-client.

The default is still to not install chef-client and assume that
the vagrant-omnibus installer will be used to install chef-client, but
if you set the user variable chef_client with the version of chef-client
desired, it will install chef-client.
Example: packer build -var 'chef_version=11.8.0-1' win2012r2-standard.json
Permit optional install of chef-client - Windows

The default is still to not install chef-client and assume that
the vagrant-omnibus installer will be used to install chef-client, but
if you set the user variable chef_client with the version of chef-client
desired, it will install chef-client.
Example: packer build -var 'chef_version=11.8.0' win2012r2-standard.json
Deleting netboot version of centos510, will be solving this issue a different way
Fixing 32-bit CentOS templates to use a 32-bit guest os type
Per Brian Flad <bflad417@gmail.com>, switching chef_version toggle to be
provisioner/provisioner_version:

* Add provisioner variable and PROVISIONER environment variable
* Rename chef_version variable to provisioner_version
* Rename CHEF_VERSION environment variable to PROVISIONER_VERSION
* Rename chef.bat script to provisioner.bat
* Use PROVISIONER to determine action in provisioner.bat
Deleting old chef.bat files left behind
Fixing typo in user variable macro reference