add source_key parameter

When `source_key` is specified data is copied directly out of the data hash -
`data[@resource[:source_key]]` into the target resource, bypassing template
evaluation.

This has uses for sourceselect => all file resources.
order the test resources

'natural' catalog order flipped somewhere between 2.7 and 3.0 (then flipped
back in 3.1 briefly - https://travis-ci.org/richardc/puppet-datacat/builds/9432821 )

For a more robust test, specify the merge order of the fragment resources.
Merge branch 'source_key'
Note the user-visible change