[#17] Added the configuration parameter 'use-short-cache-path'.

[renkin]

Additionally to the configuration parameter 'tmp', this parameter is helpful to avoid too long cache paths under Windows.

This is achieved by shortening the cache path part of the Puppet forge URI to the 7 first digits of its SHA1 checksum (like short Git commit hash approach).

Under Windows this shortening is especially important when you use an alternative Puppet forge with a quite longer URI.

[uhanisch]

Would love to see it in master branch

[ekohl]

At first sight it makes sense but this could use some tests to verify we don't accidentally break it later. Also wondering if we should use sha256 instead since security tightened systems might disallow sha1.