Improve handling of Ruby based bindings

This improves the support for Ruby based bindings by:
- doing late evaluation of the named bindings (when included) instead
  of when loaded.
- scope is available as parameter to the block given to newbindings.
- Access to evaluating parser in BindingsFactory to allow convenient
  binding of late evaluating expressions (as opposed to producing
  the result while defining the binding). #puppet_string, and 
  #puppet_expression.

Contains test that exercises the full scope (generating a puppet
interpolation expression and binding that).
Rename hiera_config.yaml to hiera.yaml

This is what hiera1 also uses by default. The hiera_config is the name
of the setting, not the file in hiera1.
Make osfamily have higher prio than environment
Fix errors in diagnostics output from BindingsComposer

It was issuing diagnostics to acceptor instead of diagnostics
Fix problems with confdir scheme (typos and broken)

The confdir scheme was broken, and it was not missing in the defaults.
Problems found when adding it are now fixed.

The functionality now is that confdir:/default?optional is included.