Removed File classes for /usr/local and /usr/local/sbin

These directories can be assumed to exist based on the Filesystem Hierachy Standard.  Its wrong to try and 'ensure' them here because a) they might conflict with people (also incorrectly) 'ensuring' them in other modules/manifests b) in our configuration they create a circular depedency loop between stages
Refactored the use of parameters

The puppet guide offers a best-practice pattern for the use of parameters for classes:
http://docs.puppetlabs.com/guides/parameterized_classes.html
I've refactored to follow this.  It basically involves inheriting from your params class rather than using dynamic variable lookup.
Merge pull request #1 from ucam-cl-dtg/param-refactor

Param refactor and removed /usr/local, /usr/local/sbin check