Fix/master/interpolate vars in hash keys

The needs for this appeared while using create_resources(), which
expects a hash, and takes hash keys as resources titles.
When using hiera, hash keys where not interpolated before this 
patch.

You can now write this into hiera and have %{hostname} properly replaced by its facter value:

    bacula::job:
        %{hostname}_Cyrus:
            fileset: MailServer
            bacula_schedule: 'CycleStandard'
        %{hostname}_LDAP:
            fileset: LDAP
            bacula_schedule: 'CycleStandard'

Similar to https://github.com/puppetlabs/hiera/pull/123 but with rspec test + update from upstream