Interpolate facts with a facts. or trusted. scope #17

[oneiros]

Variables of the form ${::var} and ${facts.var} should now result in the same interpolation.

[oneiros]

I updated the code according to what we spoke about and noted over in #17:

With this change, hdm only interpolates variables with a facts. or trusted. scope. In both cases leading colons (::) are optional, so both %{::facts.hostname} and %{facts.hostname} will give the same result.

Variables with a facts. scope are expected to be top-scope variables in the facts hash returned by PuppetDB, while variables with a trusted. scope are expected to be nested under the "trusted" key in the facts hash.

[oneiros]

Please see https://github.com/betadots/hdm/blob/issue-17/test/models/hiera_data/interpolation_test.rb for examples of the new behavior.