[ISSUE-185] net_fetcher windows platform improvement.

NOTE: I am not sure how to stub the platform in net_fetcher_spec.

Because the omnibus cookbook sets up 7-zip, and 7-zip can handle
decompression of all of the supported Unix archive formats.  On
Windows, it should be safe to defer to 7-zip for omnibus building purpose.

Also, if someone is downloading .7z files on a non-Windows platform then
one would use 7z and not 7z.exe.