Support inline content in file instruction

[nbraud]

Description

In file instructions, accept a content attribute which contains the file's desired contents, as a string:

file:
  path: /etc/foo
  contents: foo

That attribute should be mutually-exclusive with variable_name.

Context

Currently, the only (documented) way to use a file instruction is rather verbose for simple cases:

env:
  FILE_CONTENTS: foo
file:
  path: /etc/foo
  variable_name: FILE_CONTENTS

Anything else

file.contents could support shell-style variable expansion, but that isn't necessary for simpler use-cases.

[edigaryev]

Sorry for the premature closure, this feature is deployed now and is fully working 🚀