Fixes inheritance so parameters can be set by caller

Currently variables set during instantiation are lost when the config
subclass inherits from the params subclass.  Variables are available
in the template without the scope_lookup.

Matches pattern set in https://github.com/puppetlabs/puppetlabs-ntp

Fixes #1
Merge pull request #3 from croomes/fix_inheritance

Fixes inheritance so parameters can be set by caller