Update to make use of Backend.datasourcefiles()

[mattkenn4545]

This was included in Hiera 1.3.3 and should increase performance when traversing the hierarchy. I will try to do the change myself but I can't Ruby ;).

See puppetlabs/hiera@37bb93b

[gtmtech]

Not sure how we'd make it backwards compatible with Hiera <1.3.3 as well. I guess we'd have to try calling the datasources function and doing something else if it doesn't exist..

[Dan33l]

it can be reopened if it's reproducible on the latest Puppet 4 or newer