Fix inconsistent json output

The file module would in some cases return dest=/path/to/file, and in other cases return path=/path/to/file, this change makes it more consistent and in line with all the other file-related modules (i.e. copy, assemble, template, ...) by using dest= instead of path= as the input and output argument. (Of course, path is still there as an alias for compatibility).
Merge pull request #3516 from dagwieers/file-dest

Fix inconsistent json output in file module