file.content should support loading from external files

[ryane]

Inline content can become hard to work with when dealing with larger files (config files, for example). The module should be able to load a template from a file on disk.

[BrianHicks]

This is part of a larger arc of "being able to load in content from external files".

I'm seeing a couple steps here:

1. implement a {{file "filename"}} template function
2. use that call to create a query node that just has that info

But it's got to be in the graph, so it's a little more work than "here read this", which is why we haven't added it yet.

[BrianHicks]

Update from planning meeting discussion: this makes sense as a part of the file download module, #250

[pavel-main]

@ryane Hey there, a temporary workaround can be done by pre-fetching hairyhenderson/gomplate and executing it inside task resource